Contact Forms: Add `process_submission` tests

`Grunion_Contact_Form::process_submission` tests, seeing that correct stuff hits
the database after processing contact form submissions and that correct email
is sent.
Contact Forms: Add `grunion_delete_old_spam` test.

Testing deleting old spam with a generated spam feedback.
Contact Forms: Improved comments in tests.
Contact Forms: More whitespace in tests.
Contact Forms: Tests for spam handling.

Tests to see that spam is handled correctly when WP_Error occurs in spam
checking, when marked as spam and spam is not supposed to be sent, and when
marked as spam and is still supposed to be sent.
Contact Forms: More coverage in spam cleanup tests

Spam cleanup tests don't only test for deleting old spam, but also test that
new spam isn't cleaned up.
Merge pull request #675 from rase-/add/tests

Contact Forms: Added some tests