Introduce `crossplane test lint`

This is a feature that I wanted to have in xprin but never did. The `crossplane test lint` command will parse the testsuite files the same way that the upcoming `crossplane test run` would, but without running the testcases. This way we will be able not only to profit by the lint command, but also to be able to introduce the testsuite API and focus on how to discover/parse them, without actually caring about the details of the testexecutor.