config: make sure tests check for loop device support

Several tests happen to make use of loop device support without the
requisite pre-test checks. This results in spurious failures for systems
that might not have loop device support. Add _require_loop checks to
shared/298, xfs/206 and xfs/259.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/shared/298

@@ -32,6 +32,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 
 _supported_fs ext4 xfs
 _supported_os Linux
+_require_loop
 _require_fstrim
 _require_xfs_io_command "fiemap"
 _require_fs_space $TEST_DIR 307200

tests/xfs/206

@@ -53,6 +53,7 @@ _cleanup()
 # Modify as appropriate.
 _supported_fs xfs
 _supported_os IRIX Linux
+_require_loop
 
 bitsperlong=`src/feature -w`
 if [ "$bitsperlong" -ne 64 ]; then

tests/xfs/259

@@ -41,7 +41,7 @@ trap "_cleanup ; exit \$status" 0 1 2 3 15
 # real QA test starts here
 _supported_fs xfs
 _supported_os Linux
-
+_require_loop
 _require_math
 
 testfile=$TEST_DIR/259.image