Give the test suite the memory it now needs

The full run died with "Premature end of PHP process" on machines whose
php.ini keeps PHP's usual 128M: no single test needs more, but the
panel tests boot Filament again and again in one process, and the
suite outgrew it. Always in a panel test, which is why it looked like
one of them was flaky. Stated in phpunit.xml so it runs the same
everywhere.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

phpunit.xml

@@ -18,6 +18,10 @@
         </include>
     </source>
     <php>
+        <!-- The suite, not any one test, outgrew PHP's usual 128M: the panel
+             tests boot Filament again and again in one process. Stated here so
+             it runs the same on every machine, whatever its php.ini says. -->
+        <ini name="memory_limit" value="512M"/>
         <env name="APP_ENV" value="testing"/>
         <env name="APP_MAINTENANCE_DRIVER" value="file"/>
         <env name="BCRYPT_ROUNDS" value="4"/>