minor #38798 Fix transient tests on branch 4.4 (jderusse)

This PR was merged into the 4.4 branch.

Discussion
----------

Fix transient tests on branch 4.4

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Fix transient tests on branch 4.4.

I'll keep this PR open few days to watch travis/GHA for transient tests

Commits
-------

676b8080a5 Fix transient tests

src/Symfony/Component/Messenger/Tests/Transport/AmqpExt/AmqpExtIntegrationTest.php

@@ -161,6 +161,9 @@ class AmqpExtIntegrationTest extends TestCase
         $signalTime = microtime(true);
         $timedOutTime = time() + 10;
 
+        // wait for worker started and registered the signal handler
+        usleep(100 * 1000); // 100ms
+
         // immediately after the process has started "booted", kill it
         $process->signal(15);
 

src/Symfony/Component/VarDumper/Tests/Dumper/ServerDumperTest.php

@@ -89,6 +89,6 @@ DUMP
             'VAR_DUMPER_SERVER' => self::VAR_DUMPER_SERVER,
         ]);
 
-        return $process->setTimeout(9);
+        return $process->setTimeout('\\' === \DIRECTORY_SEPARATOR ? 19 : 9);
     }
 }