[6.1] Include the path to the platform's PrivateFrameworks directory in DYLD_FRAMEWORK_PATH when launching test runners

[stmontgomery]

• Explanation: This modifies the logic which sets the value of the DYLD_FRAMEWORK_PATH environment variable when launching test runners on Darwin when Xcode is installed: currently it includes the path to the platform's developer "Frameworks" directory, and this change adds the sibling "PrivateFrameworks" directory as well.
• Scope: Affects tests run on Darwin only, when Xcode is installed.
• Issues: n/a
• Original PRs: Include the path to the platform's PrivateFrameworks directory in DYLD_FRAMEWORK_PATH when launching test runners #8199
• Risk: Low. Including this runtime framework search path should have no effect when running tests for any supported version of Xcode.
• Testing: I validated this works as expected using a local build.
• Reviewers: @briancroom, @grynspan, @plemarquand, @jakepetroules

Resolves rdar://142522110

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test Windows