Ignore failures of WeakHashMapTest#test_keySet in libcore tests.

The outcome of this test depends on a weak reference being
collected before a timeout, which sometimes fails.  This
test used to pass, but recent changes have made this
non-determinism more frequent.  Until we rewrite this
properly (if ever), just ignore these failures.

Bug: 25437292
Change-Id: I1f041f6dfc1318026bce966575477486207f5351