Avoid numdiff check race condition in test

Avoid numdiff check race condition in test

Use randomly generated file names for collision free (concurrent) checks
for numdiff support.

Otherwise, if a test subproject is configured concurrently (which might
happen if ctest accidentally triggers a reconfigure due to changed files
in the main project), the following error might occur:
    "The detected numdiff executable was not able to pass a simple relative
    tolerance test."