Many tests fail on Windows with Swift 6.1

[jakepetroules]

Describe the bug
The following tests hang and never complete:

• testInputSequence
• testInputAsyncSequence
• testInputSequenceCustomExecutionBody
• testInputAsyncSequenceCustomExecutionBody
• testRedirectedOutputRedirectToSequence

The following test fails with "tester.ps1 cannot be loaded because running scripts is disabled on this system. For more information, see about_Execution_Policies at https:/go.microsoft.com/fwlink/?LinkID=135170.", but it's nondeterministic and tends not to reproduce when the test is run by itself instead of as part of the whole suite.

• testPlatformOptionsCreateNewConsole

The following tests fail with exit code 1 and produce no output:

• testPlatformOptionsPreSpawnConfigurator
• testSuspendResumeProcess

To Reproduce
swift test

Expected behavior
A clear and concise description of what you expected to happen.

Environment (please complete the following information):

• Microsoft Windows [Version 10.0.26100.3624]
• Swift version 6.1 (swift-6.1-RELEASE)
• Target: x86_64-unknown-windows-msvc

Additional context
I know this package is mostly Swift 6.2-aligned, and I haven't tested with Swift 6.2 in case that makes any difference.

[jakepetroules]

Note that package func write(_ ptr: UnsafeRawBufferPointer, completion: @escaping (Int, Swift.Error?) -> Void) never calls its completion handler.

[jakepetroules]

internal func readChunk(upToLength maxLength: Int) async throws -> SequenceOutput.Buffer? never returns nil when it receives the empty buffer indicating EOF, making the AsyncSequence infinite.

[jakepetroules]

Even after fixing those two issues, closing the parent file descriptors in "cleanup" appears to cause the process to immediately exit with code 1.