Implement automatic test discovery in src/test/

[lukasz-kaminski]

As a convention, the tests are in the test directory, so they're not packaged during build time. We'd like to leverage this mechanism.

The idea is to find rego files to test and then search for their test counterparts in the test directory. Then, the tests would be run like this:

opa test src/main/resources/opa/policies/policies.rego src/test/resources/opa/policies/policies-test.rego