btrfs-progs: tests: arg override in command line

btrfs-progs: tests: arg override in command line

Lowmem mode only repairs few cases which has a beacon file
".lowmem_repairable" in the case' directory.

However, defining TEST_ENABLE_OVERRIDE=true in command line does work
in above strategy.
Because _skip_spec() in tests/common.local isn't interpreted by shell
in that case.

Solve it by making _skip_spec() always be defined in common.local.

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
[ keep the _skip_spec check ]
Signed-off-by: David Sterba <dsterba@suse.com>