unittest: just do a single JVM call avoid multiple executions of the JVM in order to avoid high startup costs. before: > mx --vm server unittest 355.86s user 2.97s system 175% cpu 3:24.21 total > mx --vm server longunittest 241.27s user 1.40s system 151% cpu 2:39.88 total > mx --vm server shortunittest 203.36s user 2.23s system 269% cpu 1:16.42 total after: > mx --vm server unittest 202.39s user 1.02s system 130% cpu 2:36.04 total > mx --vm server longunittest 187.39s user 0.97s system 126% cpu 2:28.38 total > mx --vm server shortunittest 87.39s user 0.68s system 265% cpu 33.15 total