btrfs: add a test case to verify that per-fs features directory gets updated

btrfs: add a test case to verify that per-fs features directory gets updated

Although btrfs has a per-fs feature directory, it's not properly
refreshed after new features are enabled.

We had some attempts to do that properly, like commit 14e46e04958d
("btrfs: synchronize incompat feature bits with sysfs files").
But unfortunately that commit get later reverted as some call sites is
not safe to update sysfs files.

Now we have a new commit b7625f461da6 ("btrfs: sysfs: update fs features
directory asynchronously") to properly refresh that per-fs features
directory.

So it's time to add a test case for it. The test case itself is pretty
straightforward:

- Make a very basic 3 disks btrfs
  Only using the very basic profiles (DUP/SINGLE) so that even older
  mkfs.btrfs can support.

- Make sure per-fs features directory doesn't contain "raid1c34" file

- Balance the metadata to RAID1C3 profile

- Verify the per-fs features directory contains "raid1c34" feature file

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
[ Update commit log. Remove commented code. Add _fixed_by_kernel_commit.
  Check mkfs status. Add sync. ]
Signed-off-by: Anand Jain <anand.jain@oracle.com>