qunit: Remove race condition in 'test.mediawiki.qunit.testrunner'

Tests should be atomic because when re-running failed tests, they
run out of order (by design).

The thing being asserted here wasn't all that useful as that is
upstream behaviour we ought to be able to rely on.

Change-Id: I2f006e1cf05f5535e8220ca8ffc2da603745d184
Merge "qunit: Remove race condition in 'test.mediawiki.qunit.testrunner'"