Set FLUTTER_TEST when spawning flutter_tester (#18907)

Otherwise on Windows these will fail as an unsupported platform when things try to read defaultTargetPlatform in flutter apps.

packages/flutter_tools/lib/src/tester/flutter_tester.dart

@@ -150,7 +150,10 @@ class FlutterTesterDevice extends Device {
       printTrace(command.join(' '));
 
       _isRunning = true;
-      _process = await processManager.start(command);
+      _process = await processManager.start(command,
+      environment: <String, String>{
+        'FLUTTER_TEST': 'true',
+      });
       _process.exitCode.then((_) => _isRunning = false);
       _process.stdout
           .transform(utf8.decoder)