Delete RuntimeTest.test_gc.

This test is flaky (see below), and it's not testing anything useful.

- the value returned by totalMemory() can increase if the heap expands
- similar issue with freeMemory(),  which is only measured against the
  current heap size
- the test assumes that there aren't any other threads in the process
  allocating objects.

bug: 17663212
Change-Id: I43fb4c2d4412033cd01bb3a533a34e47905ff229