conf-sanity 15: some testbed (cnbuild) using source tree but also have

luster installed, which cause confusion of which lconf/lmc/lctl will be
used. We put $LUSTRE/utils and $LUSTRE/tests at the head of $PATH env
to ensure the tools in source tree will be searched at first. This should
not affect the tests by install, but if we got problem we'll back this
commit out.