minor #18269 [Mailer] Add new email testing functions (johanadivare)

This PR was merged into the 6.4 branch.

Discussion
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[Mailer] Add new email testing functions

Refs symfony/symfony#50200

Commits
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1904ba2 [Mailer] Add new email testing functions (fixes #50200)

Author: javiereguiluz

testing.rst

@@ -1035,6 +1035,9 @@ Mailer Assertions
     Asserts that the given address header equals the expected e-mail
     address. This assertion normalizes addresses like ``Jane Smith
     <jane@example.com>`` into ``jane@example.com``.
+``assertEmailSubjectContains(RawMessage $email, string $expectedValue, string $message = '')``/``assertEmailSubjectNotContains(RawMessage $email, string $expectedValue, string $message = '')``
+    Asserts that the subject of the given email does (not) contain the
+    expected subject.
 
 Notifier Assertions
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