Avoid numdiff check race condition in test
Avoid numdiff check race condition in test

Do not remove the test files to avoid a race condition if
cmake decides to rerun in a concurrent situation. Otherwise this would
trigger the following error:
"The detected numdiff executable was not able to pass a simple relative
tolerance test."

sairajat/dealii,naliboff/dealii,naliboff/dealii,naliboff/dealii,naliboff/dealii,sairajat/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii,sairajat/dealii,naliboff/dealii,naliboff/dealii,sairajat/dealii,sairajat/dealii