Nondeterministic test failure

[dabrahams]

I've seen this happen several times (without parallel testing enabled FWIW):

Test Case '-[LibraryTests.LibraryTests test_compileAndRun_OptionalTests]' started.
/Users/dave/src/hylo/Tests/LibraryTests/TestCases/OptionalTests.hylo:1: error: -[LibraryTests.LibraryTests test_compileAndRun_OptionalTests] : success was expected, but processing failed with thrown error: NonzeroExit(
  terminationStatus: 6,
  standardOutput: "/Users/dave/src/hylo/Tests/LibraryTests/TestCases/OptionalTests.hylo:10: precondition failure\n",
  standardError: "",
  commandLine: ["/var/folders/f3/48cjbvxd0sx81ld0qm__zqrc0000gn/T/83CF26FA-3963-4623-AE36-1256AC0165C9"])
Test Case '-[LibraryTests.LibraryTests test_compileAndRun_OptionalTests]' failed (6.646 seconds).

It's just this one test, as far as I can tell.