[4.2] [build-script] Updated build-script-impl to be able to run the tests for the new PlaygroundLogger.

[cwakamo]

Switched to using build-for-testing/test-without-building for the PlaygroundLogger tests.
The new PlaygroundLogger tests are now a proper XCTest unit test bundle, so it needs to be done this way (instead of building the test runner and then just invoking that).

This pulls the changes for #14821 into swift-4.2-branch, and it depends on apple/swift-xcode-playground-support#28.

This is part of rdar://problem/41868882.

[cwakamo]

Please test with the following PR:

apple/swift-xcode-playground-support#28

@swift-ci Please test

[cwakamo]

Please build with the following PRs:

apple/swift-xcode-playground-support#28

@swift-ci Please Build Toolchain macOS Platform

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - c347ea9

Install command
tar -zxf swift-PR-17861-48-osx.tar.gz --directory ~/

[cwakamo]

@zisko @Rostepher If one of y'all wouldn't mind re-reviewing this patch for Swift 4.2 nomination, I'd greatly appreciate it!

[zisko]

LGTM!

[cwakamo]

• Explanation: This PR adds support for building and testing the new PlaygroundLogger implementation in apple/swift-xcode-playground-support#28. This will allow the PlaygroundLogger in Swift 4.2 to, among other things, include support for CustomPlaygroundDisplayConvertible.
• Scope of Issue: Affects how build-script builds and runs the tests for PlaygroundLogger. Should not impact Swift itself.
• Risk: Low, may cause temporary build instability as this and apple/swift-xcode-playground-support#28 land
• Testing: Ran PR testing and observed no regressions. Additionally, used PR testing to produce a macOS toolchain and observed that it contained the correct content.