tests: Fix running mfsmount using valgrind in tests

If USE_VALGRING is set all the binaries should be run using valgrind.
Unfortunately mfsmount was running normally because of the way it is
called which prevents bash from using mfsmount alias (aliases are used
to prepend each call with 'valgrind' with proper arguments).

Change-Id: I92888f120c6e8733dcb1135c56c4f9d427ff8ab4