Respect -Dpartest.scalac_opts when running the test suite through ANT

Used to enable -Xcheckinit (and now also -Ybackend:GenBCode) when
compiling tests in the respective jenkins builds. This is currently
broken, as you can see in [recent build logs] [1]:

    [partest] Scalac options are:  -optimise

[1]: https://scala-webapps.epfl.ch/jenkins/view/nightlies/job/scala-nightly-checkinit/2058/consoleFull