C Compiler Attribute Cleanup Test

C Compiler Attribute Cleanup Test

There is a long-known CC attribute cleanup which allows to call a custom
cleanup function when an auto-storage variable ceases to exist. We're
gonna use it for end-of-loop and leave-locked-block macros.

This commit adds a static unit test for this compiler feature to be sure
that it really does what we want. We're looking forward to the next ISO
C norm where this may finally get a nice syntax and standardization.