This CL adds unit tests for the TypeInitializerExpression AST element.
Bug: tint:11
Change-Id: I92790563c995038cc5a09afd03e84496b8f41487
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16672
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>