python test: Add sleep(0.5) into fastestmirror tests

I hit a heisen bug that causes that all mirrors got -1.0 values that basically mean that
the measurement cannot be done/was unsuccessfull) and thus mirrors were
not not sorted.
Only fix that worked was adding of sleep() into the test.
I guess that problem could be caused by Flask server startup time -
the measurement was done in time when flask wasn't be (re)started yet.