Start cleaning up tests (3/N)

Remove Source{} with ast::Builder::create<>
Use Builder helpers where possible

Change-Id: I7b3188fa8239bf11e88ff6b396a57c23d76a3152
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/35660
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 1d2afa0..025e02c 100644
--- a/src/ast/null_literal_test.cc
+++ b/src/ast/null_literal_test.cc
@@ -28,9 +28,7 @@
 using NullLiteralTest = TestHelper;
 
 TEST_F(NullLiteralTest, Is) {
-  type::I32 i32;
-  NullLiteral i{Source{}, &i32};
-  Literal* l = &i;
+  ast::Literal* l = create<NullLiteral>(ty.i32);
   EXPECT_FALSE(l->Is<BoolLiteral>());
   EXPECT_FALSE(l->Is<SintLiteral>());
   EXPECT_FALSE(l->Is<FloatLiteral>());
@@ -40,16 +38,14 @@
 }
 
 TEST_F(NullLiteralTest, ToStr) {
-  type::I32 i32;
-  NullLiteral i{Source{}, &i32};
+  auto* i = create<NullLiteral>(ty.i32);
 
-  EXPECT_EQ(i.to_str(), "null __i32");
+  EXPECT_EQ(i->to_str(), "null __i32");
 }
 
 TEST_F(NullLiteralTest, Name_I32) {
-  type::I32 i32;
-  NullLiteral i{Source{}, &i32};
-  EXPECT_EQ("__null__i32", i.name());
+  auto* i = create<NullLiteral>(ty.i32);
+  EXPECT_EQ("__null__i32", i->name());
 }
 
 }  // namespace