It would be even better if this command setup phpunit.xml, in the same way composer.json is updated.

If all the following are true:

• phpunit.xml file exists.
• there is NO rector test suite

Then Rector adds the rector testsuite to phpunit.xml. The phpunit.xml diff would look a bit like this:

    <testsuites>
        <testsuite name="default">
	    <directory>tests</directory>
        </testsuite>
+       <testsuite name="rector">
+          <directory>utils/rector/tests</directory>
+       </testsuite>
    <testsuites>

Rector would report is has done this, in a similar way to it reports updating composer.json. E.g.

 [OK] We also update phpunit.xml, to add a rector test suite. You can run the rector tests by running: phpunit --testsuite rector

Resolves rectorphp/rector#8486