[testing] fix potentially parallel execution of tests in

test/material/fluidmatrixinteractions

Different tests have been writing output to the same file which can lead
to wrong (testing) results if the tests are executed in parallel.

Reviewed by Thomas.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14611 2fb0f335-1f38-0410-981e-8018bf24f1b0