add SKIP_ALL_TESTS to coverage report

tox.ini

@@ -30,4 +30,7 @@ commands =
     coverage run --source ansible_inventory_generator -m pytest {posargs}
     coverage xml
     coverage html
-    coverage report --fail-under 80
+    sh -c "if [ $SKIP_ALL_TESTS -eq 1 ]; then echo 'Skipping coverage report'; else coverage report --fail-under 80; fi"
+
+allowlist_externals =
+    sh
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