I have been told that the --xcode build isn't recommended for running tests, but I would like to know whether this is a defect or something that simply isn't supposed to work.
Running lit for an Xcode build (.../lit.py -sv .../test-macosx-x86_64) ends in seconds with the following message:
It is actually possible to run the tests with the Xcode build, it's just less convenient to run the entire suite.
For the unit tests there should be a target in Xcode and for the lit-based tests (which are most) use swift/utils/run-test --build-dir path/to/eg/build/Xcode-DebugAssert. It just doesn't support building all targets necessary for those tests so you would need to make sure they are up-to-date. If you don't know what might be necessary, just build ALL_BUILD though it may take some while.