Needed: variant of `assertMacroExpansion()` that does not use XCTest

[grynspan]

Description

Test targets that use swift-testing instead of XCTest need some way to work with this function; let's add a throwing equivalent that can be used without any dependency on a specific testing library.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://119519713

[ahoppen]

Implemented in #2647