_longest gets reset if aggregate_tests called multiple times

[FractalBoy]

Every time aggregate_tests is called, it calls prepare, which sets $longest to 0, finds the longest test description, and then sets the _longest attribute.

If aggregate_tests is called multiple times with the same formatter, then _longest can end up being less than the actual longest description, causing a negative repeat count warning.

The fix to this would be to start with the current value of _prepare instead of starting at 0 every time.