Make the performance tests a bit more useful

 * Remove some that weren't testing anything interesting.
 * Adjust the amount of work done in each test to get the standard deviations
   for all of the tests under 5%.
 * Only run the performance tests in the iOS Device Tests target as the speed
   in the simulator isn't very interesting and they take a while to run.
 * Add a few new tests.
Merge pull request #1006 from realm/tg-perf-test

Make the performance tests a bit more useful