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.

This gets the performance tests to a state where adding a baseline for the device plugged into the CI machine and having it complain when performance regresses too much might be practical. Main thing left to figure out is how to go about *updating* the baseline after making a change that should modify performance.