Fix unit test paths

[tdenewiler]

There is a fairly long-standing bug in tox.ini where the paths to modules are specified for coverage. The result is that only the discovery plugin coverage has been used in our Actions, not the tool plugin coverage. This was discovered while developing a new tool plugin in #15. In the Actions for that branch the coverage results for the tool plugins are less than 100%, even though running tox locally does give 100% coverage. It appears that some of the unit tests are skipped in Actions, and if those unit test run then we will get full coverage.