Fix failing minitest test on Windows

The "sets config_file from parameter, if given" test in spec/loader/yaml_spec.rb tests for '/tmp/crazyfunkytown.file'. This fails on Windows because it gets changed to 'C:/tmp/crazyfunkytown.file'.

This PR changes the must_equal to a must_match to make this test pass on Windows.

With this changed all minitest tests pass on Windows :)