`XCTestCase.validate` should be split off into its own module for use outside of `AsyncAlgorithms` tests

[phausler]

XCTestCase.validate should be split off into its own module for use outside of AsyncAlgorithms tests. The module should be called AsyncAlgorithms_XCTest so that perhaps we might be able to take advantage of cross-import overlays when/if that becomes a ratified thing for swift packages.