To test on Linux XCTestCase entries must conform to the protocol XCTestCaseProvider.
This means mostly mac developers probably won't bother making their tests run on Linux, which is not ideal.
XCTestCaseProvider will exist until Swift has reflection support on Linux, so we should try to generate these entries on Linux for now in the PM.
Use the AST to extract XCTestCase classes and their test functions, generate a source, add it to the test module.
Also generate an XCTMain for the full package suite.
This feature depends on me landing testing support (hopefully today).