Unit tests in Tests_Mail not resetting the phpmailer mock

[welcher]

While writing some tests for #33932, I found that the Test_Mail class was not reseting the phpmailer mock after the last test because the unset( $GLOBALS['phpmailer']->mock_sent ); was only happening in setUp().

[welcher]

Adding tearDown() to cleanup after last test

[SergeyBiryukov]

In 37307:

Build/Test Tools: Reset the PHPMailer mock in Tests_Mail::tearDown().

Props welcher.
Fixes #36609.