Missing tests for IniConfigParser, TomlConfigParser and CompositeConfigParser

[tristanlatr]

IniConfigParser, TomlConfigParser and CompositeConfigParser have been introduced in version 1.5.5. But without tests...

I do have the tests tho!

[bw2]

great! Should this go in after @tbooth's PR(s)?

[tbooth]

The first part of my PR is to split the tests up into more than one file. It would be easiest for me if at least this part could be accepted (or the idea of splitting up the test code rejected!) first, before test_configargparse.py gets any bigger.

I was going to do a first PR to kill off setup.py, and then a second one to split the tests. Those should be ready for review imminently as it's mostly just a matter of picking my first commits from my existing fork.