…sting tests.

This PR removes the constraint on Darwin that XCTest.framework must be present
in order to build tests using swift-testing. On Darwin, XCTest is included as a
framework inside Xcode, but if a developer installs the Xcode Command Line Tools
instead of the full IDE, XCTest is not included. They then get a diagnostic of
the form:

> error: XCTest not available: terminated(1): /usr/bin/xcrun --sdk macosx --show-sdk-platform-path output:
>    xcrun: error: unable to lookup item 'PlatformPath' from command line tools installation
>    xcrun: error: unable to lookup item 'PlatformPath' in SDK '/Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk'

Which is a poor experience if they aren't even using XCTest.

This change, as a (positive) side effect, suppresses the same diagnostic when
running commands that are not usually dependent on the presence of XCTest such
as `swift build`.

Note that swift-corelibs-xctest is not supported on Darwin, so installing the
Xcode Command Line Tools and adding an explicit dependency on
swift-corelibs-xctest will not produce a functional test target bundle.
Supporting swift-corelibs-xctest on Darwin is a potential future direction.

Automated testing for this change is difficult because it relies on a build
environment that is not supported in CI (namely the presence of the CL tools but
not Xcode nor XCTest.framework.) I have manually tested the change against
swift-testing's own test target.

A separate PR will be necessary in swift-testing to remove some remaining XCTest
dependencies. Those changes are not covered by this PR.

Resolves rdar://125372431.

Co-authored-by: Max Desiatov <[email protected]>