Add runtime tests to GitHub Actions, in addition to our existing build tests

[bimimicah]

We have tests to make sure code changes build properly on Mac, Windows, and Linux. And static security analysis,

It would be helpful to add tests for the actual functionality. Example:

• add mod_authnz_external to apache
• add mod_authz_unixgroup to apache
• set up an example authenticator/authorizer
• set example configuration directives in apache conf files
• run curl on it several times