configure: limit JunitTest VM usage

By default JVM will allocate a significant chunk of the available system
VM, but our JunitTest run well for me with just 64 MB of max heap,
so set this as a limit at least for Sun JVMs, to have tests running more
smoothly on OS that lack VM over-commit.

Change-Id: I5e4b5595decc3a133e7e285dfd7463d436fa458d
Reviewed-on: https://gerrit.libreoffice.org/11698
Reviewed-by: David Ostrovsky <david@ostrovsky.org>
Tested-by: David Ostrovsky <david@ostrovsky.org>