ci: [Policy Assistant] tests in github action

[huntergregory]

Fixes #202.

GitHub action does the following:

1. Trigger only on changes to cmd/policy-assistant/ directory
2. Run unit tests
3. Build binary
4. Run commands for each analyze mode and make sure they exit with zero code

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: huntergregory
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[huntergregory]

The GH action will fail until #244 and #245 merge.