minor #16162 [Translation] Fix tests compat with 2.8 (nicolas-grekas)

This PR was merged into the 3.0-dev branch.

Discussion
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[Translation] Fix tests compat with 2.8

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

2da72cb [Translation] Fix tests compat with 2.8

src/Symfony/Component/Translation/Tests/TranslatorCacheTest.php

@@ -288,6 +288,10 @@ class StaleResource implements SelfCheckingResourceInterface
         return false;
     }
 
+    public function getResource()
+    {
+    }
+
     public function __toString()
     {
         return '';