[test] Add missing postprocessing phase to executable stress tests
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.
(cherry picked from commit f56d82fafd047ebd224554b2e778582f33f5a198)