Enable iOS GitHub Actions workflow. (#1376)

Enable iOS GitHub Actions workflow added in
swiftlang/github-workflows#169.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

.github/workflows/pull_request.yml

@@ -21,6 +21,8 @@ jobs:
       windows_swift_versions: '["nightly-main", "nightly-6.2"]'
       enable_macos_checks: true
       macos_exclude_xcode_versions: '[{"xcode_version": "16.2"}, {"xcode_version": "16.3"}, {"xcode_version": "16.4"}]'
+      enable_ios_checks: true
+      ios_host_exclude_xcode_versions: '[{"xcode_version": "16.2"}, {"xcode_version": "16.3"}, {"xcode_version": "16.4"}]'
       enable_wasm_sdk_build: true
   soundness:
     name: Soundness