Add dedicated helper functions for test content discovery.
We're doing some refactoring of the metadata discovery logic used on ELF-based
platforms (where it's not possible to use platform API to discover Swift's
metadata sections at runtime). This change introduces a new ABI surface
specifically for test content (which is rarely present and which must be looked
up dynamically outside the runtime) so that we can eventually retire the current
logic in SwiftRT-ELF-WASM.cpp.
> [!IMPORTANT]
> The new interfaces are intended solely for use by Swift Testing and/or XCTest.
> External libraries that wish to make use of them should reach out to the Swift
> Testing team for advice; we have some experimental API for accessing this
> metadata that we may be able to offer you.
