MDL-48196 phpunit: Set e-mail address for admin user at installation
MDL-48197 testing: Always set up a mail sink for mail redirection

Rather than setting noemailever in the bootstrap, this sets up a default
phpmailer message sink which will catch all messages.

Tests which require the phpmailer sink will continue to work as before
without change, though the noemailever config setting no longer needs to
be altered.

For tests wanting to test with noemailever set with the previous behaviour,
they will need to load the message sink and then close it.
MDL-48197 testing: Normalise uses of noemailever and phpmailer sink