shared,generic: move shared/006 to generic/

The shared/006 uses _scratch_mkfs_sized to create a limited size
file system, and then creates inodes until it gets ENOSPC, and then
checks to make sure the file system is consistent.  It then remounts
the file system, removes all of the files, and makes sure the file
system is consistent afterwards. This test was marked as only being
supported on ext[234] and xfs, and so it was in shared.

Now introduce a new _require_inode_limits() rule to run test on
filesystems that have a fixed inode number, then move the test to
generic.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/group

@@ -560,3 +560,4 @@
 555 auto quick cap
 556 auto quick casefold
 557 auto quick log
+558 auto enospc