Test tools/javac/processing/model/testgetallmembers/Main.java using too small heap

The test tools/javac/processing/model/testgetallmembers/Main.java is assuming that a 256M heap is enough to hold its live-set, but this is only true under some conditions. There are a number of JVM flags that, when used, can break this assumption. For example, choice of GC, compressed oop, object alignment, and other options affecting the heap layout or allocation strategy. Under ideal conditions, the test is already fairly close to using the whole heap, so it doesn't take that much to push it over the edge. For example, the following combinations all fail:

-XX:+UseSerialGC -XX:-UseCompressedOops
-XX:+UseParallelGC -XX:-UseCompressedOops
-XX:+UseZGC (always has -XX:-UseCompressedOops)
-XX:+UseG1GC -XX:-UseCompressedOops -XX:ObjectAlignmentInBytes=16

I'd suggest we just bump the max heap size to something like 512M, to give the test more headroom and make it less sensitive to exact choice of JVM flags.