This is a step onto leveraging the `ThreadExecutor` implementation for
`ProcessExecutor` which is a follow-up to #4870. We need to have the
proper test coverage and the existing implementations working as
expected before we move to the shared code.
Fixes:
- added `--showtime=` tests for all executor implementations
- only print `--showtime=summary` once at the end
- prevents `--showtime=` by multiple threads to be written at the same
time - essentially breaking the output
- reset the timer results before each test
- deprecated `top5` in favor of `top5_file`
- fixed printing for all executors except `ProcessExecutor`
fc700b68eb