btrfs-progs: check: Fix lowmem mode override to allow it skip repair work

btrfs-progs: check: Fix lowmem mode override to allow it skip repair work

Current common.local doesn't handle lowmem mode well.
It passes "--mode=lowmem" alone with "--repair", making it unable to
check lowmem mode.

It's caused by the following bugs:

1) Wrong variable in test/common.local
   We should check TEST_ARGS_CHECK, not TEST_CHECK, which is not defined
   so we never return 1.

2) Wrong parameter passed to _cmd_spec() in test/common
   This prevents us from grepping the correct parameters.

Fix it.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>