Parallel testing

This merge request allows for tests to be run in parallel, resulting in about a 20x speedup of the test suite.

Locally, tests run to completion in under a minute, with the same coverage as shown in the previous release build process.

My local coverage (my worker.py is slightly modified, ignore the difference in statements/coverage for that one):

---------- coverage: platform linux, python 3.8.10-final-0 -----------
Name                                             Stmts   Miss  Cover
--------------------------------------------------------------------
inventory_provider/__init__.py                      46      0   100%
inventory_provider/app.py                           13     13     0%
inventory_provider/config.py                         7      0   100%
inventory_provider/db/__init__.py                    0      0   100%
inventory_provider/db/ims.py                       188     35    81%
inventory_provider/db/ims_data.py                  224     39    83%
inventory_provider/environment.py                   25      1    96%
inventory_provider/juniper.py                      168     15    91%
inventory_provider/routes/__init__.py                0      0   100%
inventory_provider/routes/classifier.py            482     87    82%
inventory_provider/routes/classifier_schema.py      13      0   100%
inventory_provider/routes/common.py                182     21    88%
inventory_provider/routes/data.py                   53      1    98%
inventory_provider/routes/default.py                22      0   100%
inventory_provider/routes/jobs.py                   82      6    93%
inventory_provider/routes/lg.py                     42      2    95%
inventory_provider/routes/lnetd.py                  55      2    96%
inventory_provider/routes/mic.py                    16      1    94%
inventory_provider/routes/msr.py                   471     58    88%
inventory_provider/routes/neteng.py                 45      2    96%
inventory_provider/routes/poller.py                503     14    97%
inventory_provider/routes/testing.py               126     37    71%
inventory_provider/snmp.py                          70      9    87%
inventory_provider/tasks/__init__.py                 0      0   100%
inventory_provider/tasks/app.py                     13      4    69%
inventory_provider/tasks/common.py                 101     29    71%
inventory_provider/tasks/config.py                  31      5    84%
inventory_provider/tasks/monitor.py                107     13    88%
inventory_provider/tasks/worker.py                 792    189    76%
--------------------------------------------------------------------
TOTAL                                             3877    583    85%

========================================================================= 738 passed, 11 skipped, 11063 warnings in 55.51s =========================================================================

The coverage from the last release build:

========= 738 passed, 11 skipped, 10978 warnings in 1261.25s (0:21:01) =========
py38: commands[2]> coverage xml
Wrote XML report to coverage.xml
py38: commands[3]> coverage html
Wrote HTML report to htmlcov/index.html
py38: commands[4]> coverage report --fail-under 80
Name                                             Stmts   Miss  Cover
--------------------------------------------------------------------
inventory_provider/__init__.py                      46      0   100%
inventory_provider/app.py                           13     13     0%
inventory_provider/config.py                         7      0   100%
inventory_provider/db/__init__.py                    0      0   100%
inventory_provider/db/ims.py                       188     35    81%
inventory_provider/db/ims_data.py                  224     39    83%
inventory_provider/environment.py                   25      1    96%
inventory_provider/juniper.py                      168     15    91%
inventory_provider/routes/__init__.py                0      0   100%
inventory_provider/routes/classifier.py            482     87    82%
inventory_provider/routes/classifier_schema.py      13      0   100%
inventory_provider/routes/common.py                182     21    88%
inventory_provider/routes/data.py                   53      1    98%
inventory_provider/routes/default.py                22      0   100%
inventory_provider/routes/jobs.py                   82      6    93%
inventory_provider/routes/lg.py                     42      2    95%
inventory_provider/routes/lnetd.py                  55      2    96%
inventory_provider/routes/mic.py                    16      1    94%
inventory_provider/routes/msr.py                   471     58    88%
inventory_provider/routes/neteng.py                 45      2    96%
inventory_provider/routes/poller.py                503     14    97%
inventory_provider/routes/testing.py               126     37    71%
inventory_provider/snmp.py                          70      9    87%
inventory_provider/tasks/__init__.py                 0      0   100%
inventory_provider/tasks/app.py                     13      4    69%
inventory_provider/tasks/common.py                 101     29    71%
inventory_provider/tasks/config.py                  31      5    84%
inventory_provider/tasks/monitor.py                107     13    88%
inventory_provider/tasks/worker.py                 786    183    77%
--------------------------------------------------------------------
TOTAL                                             3871    577    85%