Renamed config.py -> config_yaml.py in preparation for adding new config types.
Simplify test case discovery via glob() instead of listdir() + custom code.
Added missing import of the sys module, used to display errors + exit.
Moved test runners from Makefile into separate shell scripts.

This allows for easier and more extensible programmability and automation, as
we're not limited by make's shell processor and can freely use bash features,
without having to find workarounds.
Fix module path (config -> config_yaml) to unbreak test.