Restore test deliveries properly in ActionMailer.

`ActionMailer::Base.delivery_method` and
`ActionMailer::Base.perform_deliveries` have leaked states.

"delivery method can be customized per instance" and "delivery method
can be customized in subclasses not changing the parent" in
delivery_methods_test.rb will fail if test_helper_test.rb (in which
TestHelperMailerTest is inherited from ActionMailer::TestCase) runs
before it.
Merge pull request #15544 from zuhao/refactor_actionmailer_test_case

Restore test deliveries properly in ActionMailer.