tests/btrfs: use $AWK_PROG instead of awk

tests/btrfs: use $AWK_PROG instead of awk

Use $AWK_PROG instead of awk in case the default awk
program is not called awk.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Adam Manzanares <a.manzanares@samsung.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>