Adding support for testing BUILD.gn

BUG=tint:50

Change-Id: Ie19a1af6a97a49a67dcc7474991c960b986ddebe
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19782
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 9a1077b..9d2c2fe 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -402,6 +402,7 @@
     reader/wgsl/parser_impl_switch_body_test.cc
     reader/wgsl/parser_impl_switch_stmt_test.cc
     reader/wgsl/parser_impl_test.cc
+    reader/wgsl/parser_impl_test_helper.cc
     reader/wgsl/parser_impl_test_helper.h
     reader/wgsl/parser_impl_type_alias_test.cc
     reader/wgsl/parser_impl_type_decl_test.cc