[build-script] Use the correct build directory when building Darwin XCTest

[briancroom]

What's in this pull request?

Specify the XCTest build directory for both the SYMROOT and OBJROOT build settings when invoking xcodebuild. DSTROOT, which was previously being specified, is only used when performing an install action, which was disabled in #2036. With this change, all build intermediates and products end up in the correct XCTest build directory, with the system-wide DerivedData directory only receiving Xcode build logs.

This should resolve lingering issues related to leftover state from old builds causing CI failures. (See swiftlang/swift-corelibs-xctest#77 and swiftlang/swift-corelibs-xctest#95)

CC @modocache @shahmishal

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[briancroom]

Note: I just realized that I inadvertently pushed to branch to origin before making this PR, instead of to my fork. I realize this is against policy and will be more careful about doing this correctly in the future.

[rintaro]

@briancroom @modocache
BTW, why not to make swift-corelibs-xctest/build_script.py to handle Darwin build too?
I think, that reduces repository dependency, and simplify build-script-impl.

[briancroom]

Hey @rintaro, thanks for the feedback. I actually had the same thought while working on this, considering how much churn we've been introducing in the Swift build script around XCTest recently. In the past, there has been some hesitation to make the XCTest build script handle Darwin, however the intention this time around would be different, because we would be having the build script invoke xcodebuild, as Xcode is the only build tool we intend to support on Darwin. What do you think @modocache @mike-ferris-apple?

In the meantime, I would still like to see this PR merged as it should be the quickes let way to get XCTest CI working for OS X. Would someone mine kicking off a build? 🙏

[shahmishal]

@swift-ci Please test

[modocache]

I agree, after this much work it might be time to have build_script.py be the one-stop-shop for building corelibs-xctest, despite our initial reluctance. It's also a pain to send pull requests to apple/swift and hope they fix builds in apple/swift-corelibs-xctest.|

Thanks for continuing to work on this, @briancroom!! 🙇

[briancroom]

Since CI is green here, I'm going io merge this so we can get some quick feedback on a couple of open XCTest PRs. Once we have the XCTest build script changes ready, I'll make an update here to utilize it. Thanks all!