WASI does not have thread spawning method yet, but the existing
implementation blocks threads to wait async test cases synchronously.
This commit introduced a new waiter method for running async test cases in
single-threaded WASI environments, enabled by USE_SWIFT_CONCURRENCY_WAITER
flag.

With the new waiter, `XCTMain` is async runs the given test suites
without blocking the thread by bypassing some synchronous public APIs
like `XCTest.perform` and `XCTest.run`. This ignores those APIs even if
they are overridden by user-defined subclasses, so it's not 100%
compatible with the existing XCTest APIs. This is a trade-off to
support async test execution in single-threaded environments, but it
should be fine because the APIs are seldom overridden by user code.

Also this revealed teardown blocks were not being run in the mode, so
fix that as well.

To keep consistency with the regular mode.

To make the semantics of the option clearer.

The use of DispatchQueue in XCTest is now very limited, and it's only used
in a single place in XCTestCase.swift.