Update tests to use Xcode 16.2 and iOS 18.2 Simulator (#165318)

Runs tests on Xcode 16.2 and iOS 18.2. Also updates engine scenario
golden files to iOS 18.2 and removes non-impeller (Skia) test files that
we no longer use.

All framework tests passing:
https://github.com/flutter/flutter/issues/148899#issuecomment-2701465612
All engine tests passing:
https://github.com/flutter/flutter/issues/148906#issuecomment-2702112378

Fixes https://github.com/flutter/flutter/issues/148907 and
https://github.com/flutter/flutter/issues/148957.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
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[Discord]:
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[Data Driven Fixes]:
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dev/devicelab/lib/framework/ios.dart

@@ -63,7 +63,8 @@ Future<void> testWithNewIOSSimulator(
     '--json',
   ], workingDirectory: flutterDirectory.path);
 
-  // Get the preferred runtime build for the selected Xcode version. Preferred
+  // First check for userOverriddenBuild, which may be set in CI by mac_toolchain.
+  // Next, get the preferred runtime build for the selected Xcode version. Preferred
   // means the runtime was either bundled with Xcode, exactly matched your SDK
   // version, or it's indicated a better match for your SDK.
   final Map<String, Object?> decodeResult =
@@ -71,6 +72,7 @@ Future<void> testWithNewIOSSimulator(
   final String? iosKey =
       decodeResult.keys.where((String key) => key.contains('iphoneos')).firstOrNull;
   final String? runtimeBuildForSelectedXcode = switch (decodeResult[iosKey]) {
+    {'userOverriddenBuild': final String build} => build,
     {'preferredBuild': final String build} => build,
     _ => null,
   };

dev/devicelab/lib/tasks/plugin_tests.dart

@@ -370,6 +370,7 @@ public class $pluginClass: NSObject, FlutterPlugin {
           destination: 'platform=macOS',
           configuration: 'Debug',
           testName: 'native_plugin_unit_tests_macos',
+          extraOptions: <String>['-parallel-testing-enabled', 'NO'],
           skipCodesign: true,
         )) {
           throw TaskResult.failure('Platform unit tests failed');