Bug 847168 - [User Story] Email Signature

We're trying to avoid putting multiple test cases in the same file these days if we can avoid it.  This is mainly since it lets us have every test case have separate test state and decrease our cognitive burden.  So let's do that.

Let's also add a brief comment block that provides an overview of what the test does.  For example, the following would work for me:  Test that MailSendIdentity.modifyIdentity correctly lets us manipulate the signature settings and that those correctly affect the template string created by the compose process.  This does not test reply/forward permutations, but we already have existing coverage in test_compose and that logic is simple/reliable enough that we don't need the extra permutations.