Disable test where FXC emits bad output

Bug: tint:904
Change-Id: Ibefd0d83914e39b5a2db942153f183e6f334fcc8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/54880
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/tests/DawnTest.cpp b/src/tests/DawnTest.cpp
index e01d167..530b464 100644
--- a/src/tests/DawnTest.cpp
+++ b/src/tests/DawnTest.cpp
@@ -819,6 +819,10 @@
     return gTestEnv->RunSuppressedTests();
 }
 
+bool DawnTestBase::IsDXC() const {
+    return HasToggleEnabled("use_dxc");
+}
+
 bool DawnTestBase::IsAsan() const {
 #if defined(ADDRESS_SANITIZER)
     return true;