Unify tests on all supported platforms to ensure consistent behavior and add more tests.

[iCharlesHu]

This PR refactors and consolidates tests across all platforms and introduces numerous additional tests.

Now, tests are categorized into four main types:

• IntegrationTests: This serves as the primary test file that evaluates the end-to-end functionality of Subprocess by executing child processes.
• {Unix|Darwin|Linux|Windows}Tests: These tests are platform-specific and cater to the unique characteristics of each operating system.
• AsyncIOTests: These tests are dedicated to unit testing the AsyncIO functionality specifically.
• ProcessMonitoringTests: These tests are dedicated to unit testing func monitorProcessTermination specifically

These newly established test categories have revealed a few bugs in the implementation, which are also addressed in this PR:

• Moved the monitorProcessTermination function inside the withAsyncTaskCleanupHandler. This adjustment ensures that the teardown sequence can still be executed even if the body closure terminates prematurely, but the child process remains stuck.
• On Windows, we’ve modified the code to use STARTUPINFOEXW to ensure that only the explicitly provided file descriptors are inheritable.
• Make sure AsyncIO.shutdown is idempotent.
• Implemented executable path searching for Windows.
• Implemented arg0 override for Windows

Resolves: #97
Resolves: #100

[jakepetroules]

On Windows, we’ve modified the code to use STARTUPINFOEXW to ensure that only the explicitly provided file descriptors are inheritable.

❤️

Resolves: #100

[jakepetroules]

Should registerFileDescriptor precondition that the fd is not already in the _registration map?

[iCharlesHu]

We can, but I don't think it's necessary because epoll_ctl will fail if it's already registered so you get the same error.

[jakepetroules]

✘ Test stressTestWithLittleOutput() recorded an issue at IntegrationTests.swift:1460:6: Caught error: An error occurred within the AsyncIO subsystem: failed to add 27 to epoll list. Underlying error: UnderlyingError(rawValue: 17)

Could this be because _safelyClose does not call AsyncIO.shared.removeRegistration for the fd before closing it? Reading I/O fails because of low limit, fails, calls _safelyClose, fd is closed, fd # gets reused, tries to re-add, already in the epoll list, boom.

[iCharlesHu]

Introduced ProcessMonitoringTests to unit test monitorProcessTermination().

[jakepetroules]

Looks good to me!