Fix flakiness of mallinfo test.

The test was really only supposed to verify that mallinfo eventually
increase after allocations of certain sizes. Sometimes the mallinfo
returned decreases due to some compaction event, so don't fail in that
case.

Bug: 124060188

Test: Ran the test 2000 times (32 bit and 64 bit). It would fail within about
Test: 200 test iterations before.
Change-Id: Ie019107be163dcf275cedf4d1bc5759278def483