btrfs: Add zstd support to grub btrfs

- Adds zstd support to the btrfs module.
- Adds a test case for btrfs zstd support.
- Changes top_srcdir to srcdir in the btrfs module's lzo include
  following comments from Daniel Kiper about the zstd include.

Tested on Ubuntu-18.04 with a btrfs /boot partition with and without zstd
compression. A test case was also added to the test suite that fails before
the patch, and passes after.

Signed-off-by: Nick Terrell <terrelln@fb.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

tests/btrfs_test.in

@@ -18,6 +18,7 @@ fi
 "@builddir@/grub-fs-tester" btrfs
 "@builddir@/grub-fs-tester" btrfs_zlib
 "@builddir@/grub-fs-tester" btrfs_lzo
+"@builddir@/grub-fs-tester" btrfs_zstd
 "@builddir@/grub-fs-tester" btrfs_raid0
 "@builddir@/grub-fs-tester" btrfs_raid1
 "@builddir@/grub-fs-tester" btrfs_single