fix src directory created too early

tests/EndToEnd/PsalmEndToEndTest.php

@@ -24,6 +24,8 @@
 use function tempnam;
 use function unlink;
 
+use const DIRECTORY_SEPARATOR;
+
 /**
  * Tests some of the most important use cases of the psalm and psalter commands, by launching a new
  * process as if invoked by a real user.
@@ -47,8 +49,6 @@ public static function setUpBeforeClass(): void
             throw new Exception('Couldn\'t get working directory');
         }
 
-        mkdir(self::$tmpDir . '/src');
-
         copy(__DIR__ . '/../fixtures/DummyProjectWithErrors/composer.json', self::$tmpDir . '/composer.json');
 
         $process = new Process(['composer', 'install', '--no-plugins'], self::$tmpDir, null, null, 120);
@@ -63,6 +63,8 @@ public static function tearDownAfterClass(): void
 
     public function setUp(): void
     {
+        mkdir(self::$tmpDir . '/src');
+
         copy(
             __DIR__ . '/../fixtures/DummyProjectWithErrors/src/FileWithErrors.php',
             self::$tmpDir . '/src/FileWithErrors.php',
@@ -281,7 +283,7 @@ private static function recursiveRemoveDirectory(string $src): void
         $dir = opendir($src);
         while (false !== ($file = readdir($dir))) {
             if (($file !== '.') && ($file !== '..')) {
-                $full = $src . '/' . $file;
+                $full = $src . DIRECTORY_SEPARATOR . $file;
                 if (is_dir($full)) {
                     self::recursiveRemoveDirectory($full);
                 } else {