Unit tests currently output basic hex dumps of skia objects even though we have ostream operators for many of the common ones. It looks like the operators were created in the wrong namespace (flutter/testing rather than the std namespace). Changing the namespace for the associated `assertions_skia` source files makes them compatible with unit tests that deal with these objects provided that the test files include that header and gn dependency, as verified with the `display_list_unittests` here.
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