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.
This improves 'ninja check-swift-validation' performance by about 2.9x
on my Linux desktop. Also, move them to validation-test where the other
long tests live.
