[test] Make sure the test program in GetErrcMessages.cmake exits normally.

If for some reason the test program does not exit normally it'd currently lead to a false positive and it's stdout output being assigned to the output variable.

Instead, check the test program exited normally before assigning the process output to the out variable.

Follow up on rGaf2796c7
Fixes an issue discovered post commit in https://reviews.llvm.org/D98278