Rewrite the ConfigFileBackedConfigProviderTest.

(aka the "IdentityTest")

Make it a lot less dumb, use a virtual FS so we can test reloading
and saving. Fix a bunch of bugs the tests highlighted.

Only things not really tested are the global config specific
behaviours.

Change-Id: I96448eb9c401ce864cd6f39fd95088a4a8a09727
Reviewed-on: http://gerrit.dmdirc.com/4015
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>