This CL adds tests for the identifier expression AST node. Bug: tint:11 Change-Id: Ie7dd4f1de3d8a23ac7e878285564d8c5fc9e1250 Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16502 Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>