> tldr; It's useful to run 1 or more tests multiple times when debugging
and I'd like to propose an option to do so with parallel_tests

I often find that I'd like to run 1+ files locally, in parallel, to rule
out certain flakey test behaviors (order tests are being run). Kind of
like a form of "test fuzzing" I guess? The purpose is to generally rule
out the test is flakey.

I often find myself doing something like this in zsh:

    repeat 10 bin/rspec path/to/my_spec.rb

Sometimes it's more than 10, and sometimes it's more than 1 file I'd
like to run at the same time.

I'd like to do this with parallel_tests instead and I'd like to propose
this PR as a potential solution (I didn't see this functionality present
with the current options)

Example usage:

```
export TEST_FILES="
spec/test_1.rb
spec/test_1.rb
spec/test_1.rb
spec/test_1.rb
spec/test_1.rb
"

bin/spring parallel_rspec -n 5 --allow-duplicates -- $(echo $TEST_FILES)

bin/spring parallel_rspec -n 5 -- $(echo $TEST_FILES)
```

There's one _minor_ caveat:

These duplicates will still be unique per process because RSpec _also_
calls uniq in a couple places

[here](https://github.com/rspec/rspec-core/blob/1e661db5c5b431c0ee88a383e8e3767f02dccbfe/lib/rspec/core/configuration.rb#L2202), [here](https://github.com/rspec/rspec-core/blob/1e661db5c5b431c0ee88a383e8e3767f02dccbfe/lib/rspec/core/configuration.rb#L2222), and [here](https://github.com/rspec/rspec-core/blob/1e661db5c5b431c0ee88a383e8e3767f02dccbfe/lib/rspec/core/configuration.rb#L1636)

If there is interest in this PR, I'd be happy to propose a similar
PR to the RSpec team.

One other thought: I was also thinking a companion `--repeat <Integer>`
flag could be useful, where it multiplies the file list N times
and then builds the commands.

    bin/spring parallel_rspec --repeat 100 -- path/to/my_spec.rb

Would happy to hear any thoughts or opinions on that as well. I'd
be happy to do a separate PR

Co-authored-by: Michael Grosser <michael@grosser.it>