Add confirm form override_settings for tests

Some of existing tests depend on confirm form settings being set to
default values. Since in test_settings those setting are not present
default values are used and travis tests pass, but if developer would
run tests in his/hers project with non-default values of settings tests
would fail. So add override_settings(NEWSLETTER_CONFIRM_FORM_...) to
some tests to make sure each test is run with proper settings.