troubleshoot Github Actions to get runs to work on ubuntu-latest

[NickleDave]

After the refactor of unit tests in #339 to fix #330, not all runs are passing on GitHub Actions (but some are, yay!)

I believe this is do to an issue with running out of disk space, because it only happens to the ubuntu-latest runs.
See https://github.community/t/action-fails-after-20-minutes-without-log/18432/2 and
actions/runner-images#709 (comment)

I tried one of the workarounds (using apt-get clean) but that did not completely fix it.
Basically I cut and pasted this code from
GraysonWu/antrea@6f8614f

    - name: Free disk space
      # https://github.com/actions/virtual-environments/issues/709
      run: |
        sudo apt-get clean
        df -h

adding an

        if: matrix.os == 'ubuntu-latest'

Not clear to me if we run out of space because we download the test data once for each run?
Is space limited across all runs?
Or is there a clean up before/after each run?

It's super tedious but one way to troubleshoot would be to make a quick PR that scales down the runs matrix and add back items one-by-one until it crashes, to try to get at what's causing us to fail

[NickleDave]

this was fixed by #360