Remove passing tests from libcore_failures.txt.

Passing tests that are suppressed show up as warnings in vogar.  I ran
a script to look at the build logs for the buildbot and find the warnings
that are common to all recent runs, meaning the test passes in every
configuration we use, so it should be safe to enable.

Bug: 21722374
Test: looked at build logs of buildbots
Change-Id: Ied157778152f08e194ab8ffc8d7a86b133dccb99