Fix a lot of notices/warnings/deprecation notes in the test directory

tests/Util/EmailerSpy.php

@@ -8,6 +8,18 @@ class EmailerSpy extends Emailer
 {
 	public static $MAIL_DATA;
 
+	/**
+	 * Wrapper around the mail() method (mainly used to overwrite for tests)
+	 * @see mail()
+	 *
+	 * @param string $to         Recipient of this mail
+	 * @param string $subject    Subject of this mail
+	 * @param string $body       Message body of this mail
+	 * @param string $headers    Headers of this mail
+	 * @param string $parameters Additional (sendmail) parameters of this mail
+	 *
+	 * @return bool true if the mail was successfully accepted for delivery, false otherwise.
+	 */
 	protected function mail(string $to, string $subject, string $body, string $headers, string $parameters)
 	{
 		self::$MAIL_DATA = [