minor #18194 [travis] Run real php subprocesses on hhvm for Process component tests (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[travis] Run real php subprocesses on hhvm for Process component tests

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Let's see if this speeds up hhvm test matrix line.

Commits
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c722e35 [travis] Run real php subprocesses on hhvm for Process component tests

.travis.yml

@@ -13,6 +13,7 @@ addons:
 env:
     global:
         - MIN_PHP=5.3.3
+        - SYMFONY_PROCESS_PHP_TEST_BINARY=~/.phpenv/versions/5.6/bin/php
 
 matrix:
     include:

src/Symfony/Component/Process/Tests/ProcessTest.php

@@ -30,7 +30,7 @@ class ProcessTest extends \PHPUnit_Framework_TestCase
     public static function setUpBeforeClass()
     {
         $phpBin = new PhpExecutableFinder();
-        self::$phpBin = 'phpdbg' === PHP_SAPI ? 'php' : $phpBin->find();
+        self::$phpBin = getenv('SYMFONY_PROCESS_PHP_TEST_BINARY') ?: ('phpdbg' === PHP_SAPI ? 'php' : $phpBin->find());
         if ('\\' !== DIRECTORY_SEPARATOR) {
             // exec is mandatory to deal with sending a signal to the process
             // see https://github.com/symfony/symfony/issues/5030 about prepending