Benchee Test split

[PragTob]

This is a small follow up/adjustments to #483

On my laptop this gains us some seconds, less than I thought but probably still worth doing:

Before:

tobi@greeny:~/github/benchee(main)$ mix test
Running ExUnit with seed: 121632, max_cases: 32
Excluding tags: [needs_fast_function_repetition: true]

....................................................................................................................................................................................................................................................................................................................................................................................................................................................................*..........................
Finished in 10.1 seconds (8.0s async, 2.1s sync)

Result: 478 passed (138 doctests, 340 tests), 1 skipped, 4 excluded

After:

tobi@greeny:~/github/benchee(improve/benchee-test-split-480)$ mix test
Running ExUnit with seed: 683175, max_cases: 32
Excluding tags: [needs_fast_function_repetition: true]

.................................................................................................................................................................................................................................................................................................................................................................................................................................................................*...............................
Finished in 7.6 seconds (3.9s async, 3.7s sync)

Result: 480 passed (138 doctests, 342 tests), 1 skipped, 4 excluded

(took fastest out of 3 for both, good enough for now not a full benchmark I know)

The higher sync time is confusing, but I think essentially they should have always been sync but ran late enough in the file so they were the only test running and so never failed, so were technically async.