Persist mail configuration changes to storage.

[jhedstrom]

• Fixes Using module override for mail collection doesn't fully work #181

[jonathanjfshaw]

I'm concerned about something here

+    $config = \Drupal::configFactory()->getEditable('system.mail');
+    $data = $config->getRawData();
+
+    // Save the original values for restoration after.
+    $this->originalConfiguration['system.mail'] = $data;

If startCollectingMail is called twice before stopCollectingMail is called (and I think this might happen in the parrallel behat extension PR), then test_mail_collector will end up being stored in $originalConfiguration and persisted.

Maybe we need to wrap the above hunk in
if (!isset($this->originalConfiguration['system.mail'])) {}

[jhedstrom]

Yeah, I think that change would make sense.