This CL adds tests to the SwitchStatement AST element. Bug: tint:11 Change-Id: I3e12667d14ab31cc15f6ea8595847f9daf55dc46 Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16682 Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>