lints: better test utils, avoid accessing lint.Lints directly

[cpu]

This is a follow up from #337 addressing the use of lint.Lints throughout the unit tests. This in turn will unblock switching lint.Lints to be unexported which will give the project more freedom to restructure for #344.

At a high level:

• I removed the testlint/testDef directory. I'm not sure what it was used for historically but it appears unused today.
• I flattened the testlint/testCerts directory into just testdata/ at the root of the project. All of the test files are certs and I think this is a little bit cleaner.
• I moved the existing test helpers from util/testTools.go to a dedicated test package. That makes it clearer that only unit tests should be consuming things from that package vs util which is used by lint logic.
• I reworked the test helpers, adding a new test.TestLint function that is used by the vast majority of the unit tests to load a cert from the test dir and execute a lint with a given name. Presently this function uses the lint.Lints map directly but now we have 1 place to update when that field is no longer exported instead of 200+.

This is a big diff, but it should be a functional no-op (only unit tests and test data are changed).

[cpu]

Files changed 844

😭 Sorry @zakird!