Break up `unit` the test monolith

[bjaglin]

Having all tests in a single project brings challenges

• It's unclear which ones rely on testsInput/shared (see https://github.com/scalacenter/scalafix/pull/1528/files#r846795199)
• The suite is quite long, especially because of the prerequisite of ScalafixSuite (cross-publishing other modules)

Suggestion

• Many of them can distributed in existing projects, colocated with the SUT
• Integration tests between cli/reflect/core, etc could be in a given module/configuration
• Integration tests depending on shared & testInputs could be in another one