Zip and upload Xcode xcresults on scenario test failure (#55093)

The `FLUTTER_ENGINE` has a `/` in it: `ci/ios_debug_unopt_sim`.  
```
path.join(storePath, '$iosEngineVariant.zip');
```
was resolving to `path_to_output/ci/ios_debug_unopt_sim.zip`.  `path_to_output` existed, but the `ci` directory didn't:

> zip error: Could not create output file (/Volumes/Work/s/w/ir/x/w/rc/flutter_logs_dir/ci/ios_debug_unopt_sim.zip)

Change the output zip path to `path_to_output/ci_ios_debug_unopt_sim.zip` with an underscore instead.

Fixes flutter/flutter#154956

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: jmagman

testing/scenario_app/bin/run_ios_tests.dart

@@ -382,7 +382,7 @@ String _zipAndStoreFailedTestResults({
   required io.Directory resultBundle,
   required String storePath,
 }) {
-  final outputPath = path.join(storePath, '$iosEngineVariant.zip');
+  final outputPath = path.join(storePath, '${iosEngineVariant.replaceAll('/', '_')}.zip');
   final result = io.Process.runSync(
     'zip',
     [