Re-enable test that was causing TimeoutExceptions on ARM64.

The libcore test libcore.java.lang.OldSystemTest#test_gc has
been failing with a java.util.concurrent.TimeoutException on
ART Builbot's ARM64 concurrent collector configuration.  No
longer ignore this failure to see whether recent changes
(namely https://android-review.googlesource.com/#/c/204125)
fixed the issue.

Bug: 26155567
Change-Id: I5cd3c3956cfd510bab1cb9d3b9479560f9703e6d