Don't suggest autogenerated test source roots

[Vassiliy-Kudryashov]

Description

Maven generates some roots we shouldn't suggest for tests generation.

Fixes #2068

How to test

Manual tests

Repeat issue's steps even with older IDEA/UnitTestBot versions: actually Maven marks some folders as roots after "Refresh" or touching project model with some additional dependency (Generate JUnit 5 and next time try to generate tests with TestNG: new dependency triggers Maven to mark some directtories as roots).

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.