test: add basic CI testing against PRs via new Github Action

[aaron-prindle]

This PR adds a simple Github Action that runs against PRs targeting our feature branch (validation-gen) that does the following:

• runs go mod tidy and verifies no files are changed
• runs
  • go test ./staging/src/[k8s.io/code-generator/cmd/validation-gen/code-generator/cmd/...
  • go test ./stagings/src/k8s.io/apimachinery/pkg/api/...
  • go test ./stagings/src/k8s.io/apimachinery/pkg/util/...
  • This runs our go unit tests and and additionally sets UPDATE_VALIDATION_GEN_FIXTURE_DATA=true and also verifies that no files are changed from the test fixture running
• runs hack/update-codegen.sh validation and verifies that no files are changed

Currently this PR is should be merged after #87 and rebased to get the CI to green. I've included those changes in this PR for now to get to green, I will rebase to drop those changes once they are merged

I tested this that the Github Action runs in my k8s fork's aprindle/kubernetes validation-gen branch and it runs/works, see here:
aaron-prindle#8

Screenshot from this PR against my k8s fork's aprindle/kubernetes validation-gen branch (which uses this same PR branch):

Not sure why it doesn't run on this PR, I think it might related to repo or user perms related to security.

NOTE
In creating this I caught some current test issues related to go mod tidy and the subfield go unit and fixture tests. I fixed these in this PR as well