tests: Add cleaning up valgdind's GDB server temporary files

When running valgrind with GDB server turned on (which is the default)
valgrind leaves some files in /tmp directory. On machines where tests
are run often this generates tens of thousands of files in /tmp which
makes it a bit difficult to find any file there.

Change-Id: Ia7053f7bc68445ddff83f9dfcf45d153cc1f0e3f