Look for config in parent directories

[tmr232]

In many testing frameworks, it is common to an entire directory tree of tests. With the current implementation of get_config, we need to have an approvaltests_config.json file in every directory in the hierarchy. This is annoying and error prone.

I think we should make it search all parent directories until it finds a config file. That way, a single file at the top-level of the tests will be sufficient.

[nickdelgrosso]

To add to this discussion, a better model may be to simply check for the approvaltests_Config.json file in the working directory where the test runner is run, rather than relative to the location of the test files themselves.