[WIP, DNM] Enable exit tests on Android API level 28 and newer.

[grynspan]

This PR enables exit tests on Android.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[finagolfin]

Thanks, Jonathan, will try it and let you know. 👍

[finagolfin]

Seeing a couple build errors because of @available attributes:

/data/data/com.termux/files/home/swift-testing/Sources/Testing/ExitTests/WaitFor.swift:84:58: �[1;31merror: �[1;39m'ExitStatus' is only available in Android 28 or newer�[0;0m
 �[0;36m81 |�[0;0m #elseif SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android)
 �[0;36m82 |�[0;0m /// A mapping of awaited child PIDs to their corresponding Swift continuations.
 �[0;36m83 |�[0;0m private nonisolated(unsafe) let _childProcessContinuations = {
    �[0;36m|�[0;0m                                 `- �[1;39mnote: �[1;39madd '@available' attribute to enclosing let�[0;0m
 �[0;36m84 |�[0;0m   let result = ManagedBuffer<[pid_t: CheckedContinuation<ExitStatus, any Error>], pthread_mutex_t>.create(
    �[0;36m|�[0;0m                                                          |- �[1;31merror: �[1;39m'ExitStatus' is only available in Android 28 or newer�[0;0m
    �[0;36m|�[0;0m                                                          `- �[1;39mnote: �[1;39madd 'if #available' version check�[0;0m
 �[0;36m85 |�[0;0m     minimumCapacity: 1,
 �[0;36m86 |�[0;0m     makingHeaderWith: { _ in [:] }

/data/data/com.termux/files/home/swift-testing/Sources/Testing/ExitTests/WaitFor.swift:106:68: �[1;31merror: �[1;39m'ExitStatus' is only available in Android 28 or newer�[0;0m
�[0;36m102 |�[0;0m ///
�[0;36m103 |�[0;0m /// - Throws: Whatever is thrown by `body`.
�[0;36m104 |�[0;0m private func _withLockedChildProcessContinuations<R>(
    �[0;36m|�[0;0m              `- �[1;39mnote: �[1;39madd '@available' attribute to enclosing global function�[0;0m
�[0;36m105 |�[0;0m   _ body: (
�[0;36m106 |�[0;0m     _ childProcessContinuations: inout [pid_t: CheckedContinuation<ExitStatus, any Error>],
    �[0;36m|�[0;0m                                                                    `- �[1;31merror: �[1;39m'ExitStatus' is only available in Android 28 or newer�[0;0m
�[0;36m107 |�[0;0m     _ lock: UnsafeMutablePointer<pthread_mutex_t>
�[0;36m108 |�[0;0m   ) throws -> R

Not going to spend time fixing this given the bigger SwiftPM issue, but if you want to keep iterating, happy to build it locally for you.

[grynspan]

Thanks, I'll address those and ping you when it's ready to try again.

[grynspan]

sig_t is not a portable spelling, need a custom typedef.

[grynspan]

@finagolfin I believe I've got all the availability annotations in place now.