btrfs: Test if btrfs will report false ENOSPC error balancing small metadata chunk

btrfs: Test if btrfs will report false ENOSPC error balancing small metadata chunk

This is a test case for a long existing bug, caused by
over-estimated metadata space_info::bytes_may_use.

There is one proposed patch for btrfs-progs to fix it, titled:
"btrfs-progs: balance: Sync the fs before balancing metadata chunks"

The test case itself is almost the same as btrfs/181, which uses
small files to bump the reserved space to trigger the false alert.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>