xfstests: Run all tests when nothing is specified

xfstests: Run all tests when nothing is specified

Currently when no tests or test groups are specified xfstests will
silently test nothing. Interestingly enough when test groups to exclude
are specified the rest of the tests will be run.

This commit changes that to run all possible tests (for a given file
system) when no specific tests has been specified. This matches the old
xfstests behaviour.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>