4043 Enable macOS XCTest support (cla: yes, p: path_provider, p: url_launcher, p: shared_preferences, platform-macos)

4198 Remove pubspec.yaml examples from READMEs (cla: yes, waiting for tree to go green, p: file_selector, p: google_sign_in, p: image_picker, p: in_app_purchase, p: path_provider, p: url_launcher, p: video_player, p: shared_preferences, platform-ios, platform-android, platform-windows, platform-macos, platform-linux, platform-web)

4211 [url_launcher] Replaced reference to shared_preferences with correct reference to url_launcher in README. (cla: yes, waiting for tree to go green, p: url_launcher, platform-windows, platform-macos, platform-linux, platform-web, needs-publishing)