Add hlsl writer tests to BUILD.gn

The check to add the HLSL tests if needed into the BUILD.gn test list
was missing. This CL adds it.

Bug: tint:7
Change-Id: I662bc49fb4f88543cfb9b3b315deb5a678d169b4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/26001
Reviewed-by: Ryan Harrison <rharrison@google.com>

BUILD.gn

@@ -1099,6 +1099,10 @@ source_set("tint_unittests_src") {
     deps += [ ":tint_unittests_msl_writer_src" ]
   }
 
+  if (tint_build_hlsl_writer) {
+    deps += [ ":tint_unittests_hlsl_writer_src" ]
+  }
+
   configs += [
     ":tint_common_config",
     ":tint_unittests_config",