Test: Add platform helpers

[bkhouri]

Add helpers related to the current platform to assist with conditionally running test cases.

Motivation:

Some XCTests have the following implementation, or a variation thereof

#if os(macOS)
    func testLibraryEnvironmentVariable() async throws {
       //test implementation
    }
#endif

The idiomatic Swift Testing is to use the enabled/disabled traits, thus allowing the test to be compiled. As such, create some helpers function. that will allow enabling/disabling a test using these helper functions.

Modifications:

Add helper functions and add some Swift Testing tests.

Result:

Ran the equivalent of the following and ensured there were no test-related failures

for _ in $(seq 0 100);
do
    swift test --enable-swift-testing --disable-xctest
done

Blocked by #8137
Requires swiftlang/swift#78300

[bkhouri]

Ready for review, though it may be blocked as not all pipeline builds run with Swift 6.0!

[bkhouri]

This PR is blocked by #8137

[bkhouri]

Please test with following pull request:
swiftlang/swift#78300

@swift-ci Please test

[grynspan]

Be aware: this won't work reliably for tests where the test content is only compilable on one platform or another.

[bkhouri]

Be aware: this won't work reliably for tests where the test content is only compilable on one platform or another.

@grynspan : Could you please clarify this?

[bkhouri]

thought: this test will likely need to be revisited when addressing #8121 and #8176

[bkhouri]

@swift-ci please test

[bkhouri]

@swift-ci please test

[bkhouri]

@swift-ci please test windows

[bkhouri]

@swift-ci please test windows