fix(core_test): Check /usr is mounted read-only before considering /
fix(core_test): Check /usr is mounted read-only before considering /

quantum/coreos-mantle,marineam/mantle,quantum/coreos-mantle,dm0-/mantle,mischief/mantle,glevand/coreos--mantle,glevand/coreos--mantle,dm0-/mantle,mjg59/mantle,mjg59/mantle,mischief/mantle,marineam/mantle