Trigger unit test workflow on pull_requests

[serathius]

@ptabor

[gyuho]

Nice! Can we remove Travis CI now?

[serathius]

We have data from 7 runs https://github.com/etcd-io/etcd/actions/workflows/unit-tests.yaml

• 1/7 linux-amd64-integration-4-cpu runs failed
• 1/7 linux-amd64-functional failed
  As those run only on merge they are just pure flakes.

@gyuho
Do we see similar flakes on Travis runs? Just making sure that we don't introduce degradation here.

[gyuho]

Do we see similar flakes on Travis runs? Just making sure that we don't introduce degradation here.

Rerunning

[ptabor]

I would consider keeping Coverage on travis, and removing rest from Travis soon (e.g. PR today, merged tomorrow).