Add test for assignment statement.

This CL adds unit tests for the assignment statement methods.

Change-Id: I4aea8f22b530bf43b7dfdc2b82b5414cf6590fea
Bug: tint:11
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16341
Commit-Queue: Sarah Mashayekhi <sarahmashay@google.com>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a79ceba..a246ee2 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -195,6 +195,7 @@
 set(TINT_TEST_SRCS
   ast/array_accessor_expression_test.cc
   ast/as_expression_test.cc
+  ast/assignment_statement_test.cc
   ast/binding_decoration_test.cc
   ast/bool_literal_test.cc
   ast/builtin_decoration_test.cc