Run flutter_test tests against Metal and OpenGLES backends #180601
Description
Currently, tests like fragment_shader_test.dart will only execute against the vulkan backend. flutter_tester lacks the ability to run against any other backend which is important when the test incorporates things like impellerc which behaves differently for each backend.
notes:
- while flutter_tester technically runs with the vulkan backend, it doesn't actually run the tests in question getUniformFloat(String name) fails on Vulkan #180595 will fix that