Expose the target triple as a separate string from the testing library version.

[grynspan]

This PR adds a script to our CMake build to capture the target triple being used to build Swift Testing so that we can include it in our diagnostic output.

Apple's fork of Swift Testing (as included with Xcode 16) also includes this information, and it is very useful to have when reviewing bug reports as it can tell us if a problem is specific to a given architecture or OS variant.

Checklist:

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

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@kateinoigakukun FYI some WASI-specific tweaks here.