Fix ResizeObserver SwiftPM iOS builds

[cipolleschi]

Summary:

Commit b03652af789b5e2d0d31554dc61230cd4ad9964d added the ResizeObserver implementation to the React-Fabric SwiftPM target, but did not exclude its C++ test directory.

SwiftPM therefore recursively treated ReactCommon/react/renderer/observers/resize/tests/ResizeObserverTest.cpp as a production framework source. All iOS, iOS Simulator, and Mac Catalyst prebuilt-core slices failed while compiling it because GoogleTest is not a dependency of the production React-Fabric target:

fatal error: 'gtest/gtest.h' file not found

This adds observers/resize/tests to the target's existing test exclusions, matching the treatment of the other React-Fabric test directories and leaving the dedicated C++ test target unchanged.

Changelog:

[INTERNAL] [FIXED] - Exclude ResizeObserver C++ tests from the React-Fabric SwiftPM production target.

Test Plan:

• Inspected all six failed prebuild_react_native_core jobs from run 32239670611; each failed compiling ResizeObserverTest.cpp due to the missing gtest/gtest.h header.
• git diff --check
• swift package --disable-sandbox dump-package and verified that the React-Fabric target excludes observers/resize/tests.
• A targeted local xcodebuild was attempted, but this environment prevents SwiftPM's sandbox-exec from running before dependency resolution; CI will exercise the original failing build path.

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D116607687.

[meta-codesync]

@cipolleschi merged this pull request in 8c64de2.