Don't rebuild when running tests

[dfabulich]

skip test was running swift build followed by swift test.

For some reason, swift test assumes that the build has to rerun from scratch with the flags that we're passing, so the swift build was a complete waste. (This doesn't happen when you just swift build --build-tests followed by swift test … the --xunit-output seems to be triggering it.)

Anyway, pre-building is a waste of time regardless, even if it's just a 10ms freshness check. This PR removes the pre-build step, letting swift test run the build.

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

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