tar_test: don't check for extra/missing when validating relative to "."
tar_test: don't check for extra/missing when validating relative to "."

right now, Check() doesn't check for missing/extra files. So, for the
sake of this test, we don't want to do this check for extra/missing files
when we validate the validation manifest produced from a tar against
the "." root directory (which would be /testdata).

Signed-off-by: Stephen Chung <dfcd4e1043ff9779fc85be4e40636ec800ab4d59@redhat.com>

vbatts/go-mtree,vbatts/go-mtree