Make the RedirectCommandTest work sanely.

Mock out everything that it uses, remove dependency on lots of
global state, and re-implement the actual test which was @Ignored.

Change-Id: I40c15e1a624b810a69c678a53fc0741bcec9e8b6
Depends-On: I63879db5c13517cfa60941fce73a2ba01e63f1cd
Reviewed-on: http://gerrit.dmdirc.com/2682
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager