Test single-path switch statements

Test single-path switch statements

This CL tests switch statements that only have a default case and also
an empty case that falls through into a default case.

New tests:
dEQP-GLES3.functional.shaders.switch.default_only*
dEQP-GLES3.functional.shaders.switch.empty_case_default*

VK-GL-CTS Issue: 3659

Components: OpenGL

Change-Id: I9a59582ec2829d73f590f1fe62d841b36e02e915