DKIMTest: make exception expectation more specific

As things were, the `DKIMTest::testDKIMSignOpenSSLNotAvailableException()` test _could_ potentially pass even when another `Exception` than the `PHPMailer\PHPMailer\Exception` was being thrown, as _all_ exceptions extend the PHP native `Exception` class.
Now this risk is not that high, as there is also a check on the exception message, but still.

Making the exception expectation more specific (by changing the import `use` statement), should still make the test more stable.

test/PHPMailer/DKIMTest.php

@@ -13,7 +13,7 @@
 
 namespace PHPMailer\Test\PHPMailer;
 
-use Exception;
+use PHPMailer\PHPMailer\Exception;
 use PHPMailer\PHPMailer\PHPMailer;
 use PHPMailer\Test\SendTestCase;