check: avoid spurious complaints that tests/$FSTYP/group does not exist

check: avoid spurious complaints that tests/$FSTYP/group does not exist

There are no tmpfs specific tests, so tests/tmpfs does not exist.
This commit avoids printing a spurious error message when running
specifying a group of tests (e.g., "check -g quick"):

  DEVICE: test:/tmp
  MK2FS OPTIONS:
  MOUNT OPTIONS: -o block_validity
  ./check: line 96: tests/tmpfs/group: No such file or directory
  FSTYP         -- tmpfs
  PLATFORM      -- Linux/i686 kvm-xfstests 4.5.0-rc2ext4-00002-g6df2762
  MKFS_OPTIONS  -- test:/scratch
  MOUNT_OPTIONS -- -o size=1G test:/scratch /test/scratch

  generic/001      [10:31:10][    5.811742] run fstests generic/001
      ...

Similar problems have been reported when testing nfs using xfstests.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>