We need to run utils/swift-darwin-postprocess.py on every executable test on Darwin platforms to work around a dyld issue. (And to ad-hoc sign executables on platforms that need it.)
Add a %target-codesign step to stress tests that are currently missing it.
The test might want to be split in two, because it does both checks for
the output of the compilation, and the run time itself, but for now mark
it as executable, so it doesn't break the Android test suite (which
cannot execute tests).
