Remove .github/workflows/test.yml

[moorepants]

We have tests.yml which covers a matrix of os, python, and ipopt versions. There is also test.yml that seems to cover one instance of that matrix and is redundant. I'm not sure why we have test.yml. I think it probably should be removed.

[brocksam]

The original reasons for this was that test.yml (with its single OS, Python version, and IPOPT version combination) is triggered on push and PR to all branches, while tests.yml (with the full matrix) is only triggered on PR to the master branch. The reason I did this in the first place no longer really feels valid: parallel jobs were limited to two at a time and the Windows jobs took ~15 mins, so if a test was failing because of a general mistake it could take a few hours to get that feedback via PR.

I think we can delete test.yml and change the on conditions in tests.yml to be just on: [push, pull_request].