minor #32924 [cs-fixer] Use PhpUnit native assertions in filsystem checks (jderusse)

This PR was merged into the 3.4 branch.

Discussion
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[cs-fixer] Use PhpUnit native assertions in filsystem checks

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

This PR change the minimal value of PhpUnit introduced in #32885 in branche 3.4.
Note: `5.6` is the highest value at the time.

Commits
-------

226bdd18fb Use PHPunit assertion

Author: fabpot

Tests/IntlTest.php

@@ -61,7 +61,7 @@ public function testGetIcuStubVersionReadsTheVersionOfBundledStubs()
 
     public function testGetDataDirectoryReturnsThePathToIcuData()
     {
-        $this->assertTrue(is_dir(Intl::getDataDirectory()));
+        $this->assertDirectoryExists(Intl::getDataDirectory());
     }
 
     /**

Tests/Util/GitRepositoryTest.php

@@ -51,7 +51,7 @@ public function testItClonesTheRepository()
         $git = GitRepository::download(self::REPO_URL, $this->targetDir);
 
         $this->assertInstanceOf(GitRepository::class, $git);
-        $this->assertTrue(is_dir($this->targetDir.'/.git'));
+        $this->assertDirectoryExists($this->targetDir.'/.git');
         $this->assertSame($this->targetDir, $git->getPath());
         $this->assertSame(self::REPO_URL, $git->getUrl());
         $this->assertRegExp('#^[0-9a-z]{40}$#', $git->getLastCommitHash());