TestMetaspaceAllocationMTxx: tighten commit sizes checks

TestMetaspaceAllocationMTxx test metaspace by trampling on one or multiple arenas repeatedly, trying to make it break, and afterwards checking that committed memory is within expectations. The tests need to guess a correct memory size (a value range within memory usage is considered okay) but the actual usage is a bit fuzzy since metaspace has internal overheads.

JDK-8277092 was a recent test error caused by changes to metaspace uncommit behavior (JDK-8276731). That was a harmless test issue, but it only showed up very intermittently when ideally it would have shown up more consistently. That shows that the heuristics in these tests are too lenient and need to get tightened.