PHPCS 3.6.2 added a sniff for a PSR-12 rule which was previously not strictly checked: "No blank line after the opening brace of a class".
This fixes the newly flagged issues.
* Tests/reorganize: move OAuth test to own file
This test uses the `Yoast\PHPUnitPolyfills\TestCases\TestCase` instead of the `PHPMailer\Test\TestCase` base class as it doesn't need the `set_up()` or `tear_down()` methods from the PHPMailer base test class.
* OAuthTest: various test tweaks
Minor test tweaks:
* Add `@covers` tags.
* Add "failure" messages to assertions.
* Minor comment punctuation.
Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>