swift234

[build-script] XCTest CI builds on OS X

Fix the OS X XCTest CI preset:

1. Installation of XCTest isn't supported on OS X. After all, the
   library is meant to be used on Linux, and not intended to be used or
   installed on OS X. Therefore, no longer pass an `INSTALL_PATH` to the
   `xcodebuild` invocation on OS X.
2. `xcodebuild` expects `swift-stdlib-tool` to be present in the same
   directory as the `SWIFT_EXEC` it's given. Re-use some shellscript
   from the toolchain installation script to copy the stdlib-tool
   substitute.

This allows the XCTest CI to work on all platforms.