Add swiftpm-xctest-helper rpath on macOS.

[dan-zheng]

Add an extra rpath for swiftpm-xctest-helper on macOS:
@executable_path/../../../lib/swift/macosx.

This fixes SR-12600:

$ swift test --filter a
error: signalled(6): /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-06-10-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper /Users/danielzheng/TestPkg/.build/x86_64-apple-macosx/debug/TestPkgPackageTests.xctest /var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/TemporaryFile.3eFu0u output:
    dyld: Library not loaded: @rpath/XCTest.framework/Versions/A/XCTest
      Referenced from: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-06-10-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper
      Reason: image not found

---

Fix found by @abertelrud in #2694 (comment).

[dan-zheng]

Just triggering tests.
@swift-ci Please test

[abertelrud]

This looks good to me, but I am not as familiar with how SwiftPM is built as some others are. Would also appreciate review by @aciidb0mb3r and @neonichu

[neonichu]

I don't think we should be using unsafeFlags here and instead do this in the bootstrap script.

[dan-zheng]

I don't think we should be using unsafeFlags here and instead do this in the bootstrap script.

Here's a PR (with more context) that edits the bootstrap script: #2692.

The change is more invasive though, adding the rpath to all tools instead of just swiftpm-xctest-helper.

Any suggestions would be appreciated for moving forward with this PR or that one!

[neonichu]

Oh, I didn't realise this change had so much history. It seems like Ankit and Anders think it is fine to do this with unsafeFlags so let's go ahead with this.

[neonichu]

@swift-ci please smoke test

[dan-zheng]

Could someone please help merge this PR, if everything's okay? @neonichu

[abertelrud]

Thanks! Yes, I think this is preferable to adding this flag to all the binaries.