Ability to warn about @testitem calls not in *_test.jl files

[nickrobinson251]

What if you write test-items but forget to put them in a "test file" i.e. a file with the suffix _test.jl? These test-items won't be run. It'd be good to have a built-in option to safe-guard against this, e.g. an option to parse all files in the directory tree down from the given paths and assert that non-test-files don't contain @testitem or @testsetup calls