Revise 004-NativeAllocations test.

Simplifies test logic for testing that RegisterNativeAllocation triggers
GC and doesn't deadlock.

Removes the test that RegisterNativeAllocation bounds the maximum
allocated native bytes, because we no longer wish to provide a guarantee
of that kind.

Test: ./test/testrunner/testrunner.py -b -t 004-NativeAllocations --host
Test: Verify the test fails if RegisterNativeAllocation does nothing.
Test: Verify the test fails if kNativeAllocationFinalizeTimeout is 0.

Bug: 68792448
Bug: 70831911

Change-Id: I3f6625869a89976e22918e2bca79c87f54aa9596