minor symfony#28192 [PhpUnitBridge] Add missing stderr redirection (greg0ire)

This PR was merged into the 3.4 branch.

Discussion
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[PhpUnitBridge] Add missing stderr redirection

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This seems to have been overlooked in
6c0e6af, 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.

Commits
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d67b86b Add missing stderr redirection

Author: nicolas-grekas

src/Symfony/Bridge/PhpUnit/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';