Re-enable tests that were causing TimeoutExceptions on x86/x86-64.

Some libcore tests have been failing with a
java.util.concurrent.TimeoutException on ART Builbot's
x86/x86-64 concurrent collector configurations.  No longer
ignore these failures to see whether recent changes (namely
https://android-review.googlesource.com/#/c/204125) fixed
the issue.

Bug: 26507762
Change-Id: Ib188e194195fc11b539bd53a03db78ab80626f33