Use xcodebuild for ios unit tests

[jjliu15]

Description

Update github unit test actions to use xcodebuild test instead of pod lib lint to run iOS unit tests.
This is necessary as part of supporting the register webview API,, since we need to add s.dependency 'webview_flutter_wkwebview' to our podspec. This only builds in iOS apps that have the flutter podfile macros, and not in pod lib lint.

Also fixes the google_mobile_ads_exampleTests target in the example xcodeproj so it points to the right xcframework. This lets us run the tests from xcode. For some reason this broke after updating Google-Mobile-Ads-SDK past 9.10.

Related Issues

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