Discover tests to checkers in files as well as directories

[sourya-deepsource]

Currently, for a given checker abc.yaml, we require the tests for the file to be in abc.test.<extension>, where <extension> is the language supported by the checker. This is limiting in cases that a checker might need to test for patterns in multiple files, or when the file name cannot be abc.test.<extension>.

The test runner should also look for tests inside the directory named abc.test for the test runner to be comprehensive.