Add unit tests for api-tools/parser/validator

[jpahm]

Add unit tests using Go's built-in testing library. I would personally be in favor of the test files being within the same package as the files being tested so that non-exported methods can be tested more easily; make sure to follow the _test.go file naming convention detailed in the linked docs. Be sure to seek out resources for details on how to write good unit tests if you are unsure.