Handle case where remaining groups are empty

[bburgin]

Similar to this issue mark-adams#17, but for the the pytest-split-tests module:
I want to avoid the "Invalid test-group argument" error for cases where the remaining group is empty, due to the item count and group_count yielding a group_size that does not evenly distribute across all the groups of the group_count. Example:
item count = 2001
group_count = 200
group_size is then 11
and the last 18 groups are empty
When iterating over the 18 empty groups, I don't want the pytest-split-tests plugin to fail out.
I want it to instead just run no tests for these groups.

[wchill]

Seems like #2 also was an old attempt at addressing this issue, but instead of leaving empty groups it would distribute the work better so that there were no empty groups. Wouldn't that be a better approach?

[bburgin]

This pull request is smaller and does not change the existing distribution behavior. I agree that the other pull request adds more value, but it changes more behavior. Also since the other pull request was not accepted, I figured I would attempt a simpler fix for the issue I was getting.

[rivershah]

Can this please be merged

[ekrako]

please merge it, without it it breaks testing

[rjeffman]

Any chance that this PR gets merged?