Add --schema flag to opa test

[anderseknert]

Should work identically to opa eval, but having to run a single command for e.g. TDD makes for less friction.

[renatosc]

I can take care of this

[renatosc]

Hi @anderseknert

I added the --schema flag to opa test and tested again my "merge error" repo with success.

Before the fix:

opa test regos/policy2.rego tests/policy2_test.rego --schema schemas/policy2_schema.json 
Error: unknown flag: --schema

After the fix:

opa_darwin_amd64 test regos/policy2.rego tests/policy2_test.rego --schema schemas/policy2_schema.json 
1 error occurred: regos/policy2.rego:11: rego_type_error: undefined ref: input.dept
	input.dept
	      ^
	      have: "dept"
	      want (one of): ["department"]

I didn't submit the PR yet because I still need to check where to add the proper test case for that new flag.

Meanwhile, since it is my first PR, do you mind checking my fix commit so I can adjust it if needed before I submit the PR?

Also, a few extra things that I noticed:

1. I still get the merge error if I try to run opa test . --schema schemas but I noticed that I also get the same merge error when trying to do opa eval data.policy1.allow --data . --schema schemas. As you pointed out, this is due to opa merging all files together. I was thinking here, maybe opa should automatically ignore the schema files/path for both eval and test? What do you think? I could work on this if you think it makes sense.

2. I noticed that the opa test is not reading the Metadata annotation from the rego files. Can you confirm that? If you want to open a new issue for it I can add Annotation support for opa test so it can read the schema info from the metadata (which is important when passing a schema directory instead of a schema file to opa test).

Thanks

[anderseknert]

Awesome! The commit looks fine to me, but it would be good if some tests were added just to assert this works as expected.