makefile: simplify test threads (#6524)
makefile: simplify test threads (#6524)

Signed-off-by: Jay Lee <2b53900d99d9dec83fb079f280f37f8f23b61cc6@gmail.com>
c4pt0r/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv