Replaces any non-alphanumeric characters with '-' when creating JUnit reporter filenames.

Fixes a crash when using JUnit Reporters with a `describe` that has a '/' in it.
Adds an integration test for the bug where the JUnit reporter would crash if your describe had a slash in it since that was used as a filename.

This starts an integration test framework for testing specific reporters.
See discussion in https://github.com/CapnKernul/minitest-reporters/pull/75
Merge pull request #75 from carols10cents/junit_filename_bug

Replaces any non-alphanumeric characters with '-' when creating JUnit re...