tox: generate coverage report for all the plugins (#1121) (#1130)

[PR #1121/0ee69813 backport][stable-4] tox: generate coverage report for all the plugins

This is a backport of PR #1121 as merged into main (0ee6981).
Include all the plugins in the coverage report, including those with
zero coverage currently.

Reviewed-by: Mark Chappell <None>

tox.ini

@@ -12,7 +12,7 @@ deps =
   pytest-ansible-units
   -rtest-requirements.txt
   with_constraints: -rtests/unit/constraints.txt
-commands = pytest --cov-report html --cov plugins {posargs:tests/}
+commands = pytest --cov-report html --cov plugins/callback --cov plugins/inventory --cov plugins/lookup --cov plugins/module_utils --cov plugins/modules plugins {posargs:tests/}
 
 [testenv:clean]
 deps = coverage