[Testing] Isolate ApplicationFileProcessorTest cache dir to fix parallel flake

tests/Application/ApplicationFileProcessor/ApplicationFileProcessorTest.php

@@ -20,6 +20,11 @@ protected function setUp(): void
     {
         parent::setUp();
 
+        // reset the container to boot an isolated cache dir, so parallel test processes cannot
+        // wipe the shared default cache mid-run and flip the caching assertions below
+        self::$rectorConfig = null;
+        $this->bootFromConfigFiles([__DIR__ . '/config.php']);
+
         $this->applicationFileProcessor = $this->make(ApplicationFileProcessor::class);
         $this->changedFilesDetector = $this->make(ChangedFilesDetector::class);
     }

tests/Application/ApplicationFileProcessor/config.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+use Rector\Config\RectorConfig;
+
+return static function (RectorConfig $rectorConfig): void {
+    // isolated cache dir, so parallel test processes cannot wipe the shared default cache
+    // mid-run and flip the caching assertions of this test
+    $rectorConfig->cacheDirectory(sys_get_temp_dir() . '/_rector_application_file_processor_test');
+};