[web][ios-safari] unskip pointer bindings test

[nturgut]

17 methods were skipped to enable ios-safari unit testing, for this class.
https://github.com/flutter/engine/blob/master/lib/web_ui/test/engine/pointer_binding_test.dart

They were failing for ios-safari uni tests( iOS13.0, iPhone 11)

[nturgut]

For more details on the failures logs: https://logs.chromium.org/logs/flutter/led/nurhan_google.com/652cd2c26ba579ff4178a22163dc44ab5b722aacb796f3e9cc453cf9b20b9fb3/+/steps/felt_ios-safari_test/0/stdout

[pedromassangocode]

Still valid today as the skipped test cases does exist.

[yjbanov]

We moved all our Safari tests to macOS Safari. Our previous iOS testing approach has lead to severe flakiness and maintenance issues. We should try again, but for now this is not actionable as we're not skipping any tests on iOS.