cli: add test harness for cli invocation with file resolution cases

[noahdietz]

We need to come up with a test harness for CLI invocations of api-linter. While fixing #1465 I didn't cover two edge cases effectively with local manual testing. A lot of the complexity comes from setting up protoparse APIs, ResolveFilenames and Parse, to properly resolve things with the given input, combined with a large set of possible combinations that callers can come up with.

Ideally, we have a a much more robust cmd/api-linter/cli_test.go with a lot more test data, with imported protos, and changing process directories.