ast: Have all tests derive from new TestHelper

It doesn't actually do anything (yet), but will hold the context and
a helper for constructing AST nodes.

Bug: tint:322
Change-Id: Ic7ba92bf39abf64ff2ac51d81c8a6338f5eff608
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/32663
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/ast/null_literal_test.cc b/src/ast/null_literal_test.cc
index 5e48f61..2ede9c3 100644
--- a/src/ast/null_literal_test.cc
+++ b/src/ast/null_literal_test.cc
@@ -14,14 +14,14 @@
 
 #include "src/ast/null_literal.h"
 
-#include "gtest/gtest.h"
+#include "src/ast/test_helper.h"
 #include "src/ast/type/i32_type.h"
 
 namespace tint {
 namespace ast {
 namespace {
 
-using NullLiteralTest = testing::Test;
+using NullLiteralTest = TestHelper;
 
 TEST_F(NullLiteralTest, Is) {
   ast::type::I32Type i32;