Add missing stderr redirection

This seems to have been overlooked in
6c0e6af47a5f36b906892537f5b2fbf15dab30b2, and results in the test suite
being polluted on machines where phpdbg is not installed.
I updated the code to mimic other occurences of exec in this file.

Author: greg0ire

Tests/CoverageListenerTest.php

@@ -16,7 +16,7 @@ public function test()
             $this->markTestSkipped('This test cannot be run on HHVM.');
         }
 
-        exec('type phpdbg', $output, $returnCode);
+        exec('type phpdbg 2> /dev/null', $output, $returnCode);
 
         if (\PHP_VERSION_ID >= 70000 && 0 === $returnCode) {
             $php = 'phpdbg -qrr';