dev: Make deprecations notices less verbose in tests

There are a lot of indirect deprecations notices that I can't fix for
the moment. They take a lot of space and the results of the tests are
harder to read. So I'm making them less visible with the risk of missing
some important deprecations that I could fix. It should be a temporary
fix (but we all know that "temporary" means "for an undefined amount of
time" :)).

.phpunit.xml.dist

@@ -13,6 +13,7 @@
         <ini name="error_reporting" value="-1" />
         <server name="APP_ENV" value="test" force="true" />
         <server name="SHELL_VERBOSITY" value="-1" />
+        <env name="SYMFONY_DEPRECATIONS_HELPER" value="weak" />
     </php>
 
     <testsuites>