[hlsl-writer] Move test helper to a template.

This CL updates TestHelper to be a templated class and converts the test
suites into using statements based off that template.

Bug: tint:7
Change-Id: I747434e7c6a8ecae353df448f06153e982dcec44
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/27500
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/writer/hlsl/generator_impl_array_accessor_test.cc b/src/writer/hlsl/generator_impl_array_accessor_test.cc
index ce7371d..0462efe 100644
--- a/src/writer/hlsl/generator_impl_array_accessor_test.cc
+++ b/src/writer/hlsl/generator_impl_array_accessor_test.cc
@@ -27,8 +27,7 @@
 namespace hlsl {
 namespace {
 
-class HlslGeneratorImplTest_Expression : public TestHelper,
-                                         public testing::Test {};
+using HlslGeneratorImplTest_Expression = TestHelper;
 
 TEST_F(HlslGeneratorImplTest_Expression, EmitExpression_ArrayAccessor) {
   ast::type::I32Type i32;