Unit Tests: Re-implement the mock PHPMailer

[ocean90]

Follow-up: ​#UT47

I would like to get this work again, especially for #23642.

[ocean90]

Align mock_sent and add $phpmailer to the list of global vars

[ocean90]

The change to test_wp_mail_custom_boundaries() is needed because of a change in PHPMailer 5.2.2, see ​https://code.google.com/a/apache-extras.org/p/phpmailer/source/browse/tags/5.2.2/changelog.txt#50

[SergeyBiryukov]

Replying to ocean90:

The change to test_wp_mail_custom_boundaries() is needed because of a change in PHPMailer 5.2.2, see ​https://code.google.com/a/apache-extras.org/p/phpmailer/source/browse/tags/5.2.2/changelog.txt#50

​https://code.google.com/a/apache-extras.org/p/phpmailer/source/diff?spec=svn113&r=113&format=side&path=/trunk/class.phpmailer.php#sc_svn112_1500

[SergeyBiryukov]

In 1300/tests:

Re-implement the mock PHPMailer. props ocean90. fixes #24662.