Please support multiple test_xxxx test cases in a single directory

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Let's say I develop a library with 15 .cpp files. I place all the files in a single lib/x/src directory and everything is compact, simple, and easy to follow. Now I want to unit test these and platform requires that I should place each of the 15 test_xxxx.cpp in a separate directory. This has several drawbacks:

1. Unnecessary complexity of the testing directory and burden to the users.
2. No clear correlation between the production code structure and the test code structure.
3. Difficult to tell which production files have a test and which ones are not.
4. No clear place to put testing utilities that are shared by multiple tests.

Request: Please allow to have multiple test_xxxx.cpp files in test directories. When building a specific test, compile the relevant test_xxxx file and exclude the other ones. This will simplify to your users the process of writing tests and will improve the user experience.

Start by tracing PlatformIO Core's test discovery and build entry points for test directories containing test_xxxx.cpp files, comparing them with the requested lib/x/src layout. Verify how a selected test is chosen and how other test_xxxx.cpp files are excluded. Done means multiple test files can share one directory while building only the selected test and preserving shared testing utilities.