Use base class for config file parsers

The configuration file parsers have a defined API, but we aren't
enforcing use of that. This adds an abstract base class that they
all will now inherit from. Additionally, new tests are added to
ensure the classes inherit from the new base class.

Moves tests for files in the config/ directory into their own
test directory.

Fixes the test class name for the ToxIni tests.

Change-Id: I003b4add6060075676da0f23cc37445ea50ffe88