Fixed wrong message in ConstraintTestCase

The SelfDescribing test was reporting lack of implementation of the
\Countable class not the SelfDescribing class. While not a problem, it
would be at least misleading to contributors seeing the error.

tests/unit/Framework/Constraint/ConstraintTestCase.php

@@ -36,7 +36,7 @@ final public function testIsSelfDescribing(): void
         $this->assertTrue($reflection->implementsInterface(SelfDescribing::class), \sprintf(
             'Failed to assert that "%s" implements "%s".',
             $className,
-            \Countable::class
+            SelfDescribing::class
         ));
     }