btrfs: add a test case to verify scrub speed throttle works

btrfs: add a test case to verify scrub speed throttle works

We introduced scrub speed throttle in commit eb3b50536642 ("btrfs: scrub:
per-device bandwidth control"),  but it is not that well documented
(e.g. what's the unit of the sysfs interface), nor tested by any test
case.

This patch will add a test case for this functionality.

The test case itself is pretty straightforward:

- Fill the fs with 2G file as scrub workload
- Scrub without any throttle to grab the initial speed
- Set the throttle to half of the initial speed
- Scrub again and check the speed against the throttle

The test case has an assumption that we can exclusively use all the
performance of the underlying disk.
But for cloud environment it's not ensured 100%, thus the test case is
not included in auto group to avoid false alerts.

[zlang: add auto group, remove useless comments]

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oralce.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>