Execution Tests: change warning to error (microsoft#8116)

Previously, this was a warning while we convinced ourselves that an
error wouldn't break anything.

Fixes microsoft#7958

Author: damyanp

tools/clang/unittests/HLSLExec/HlslExecTestUtils.cpp

@@ -84,8 +84,8 @@ static bool createDevice(
 
 ) {
   if (*D3DDevice)
-    LogWarningFmt(L"createDevice called with non-null *D3DDevice - "
-                  L"this will likely leak the previous device");
+    LogErrorFmt(L"createDevice called with non-null *D3DDevice - "
+                L"this will likely leak the previous device");
   if (TestModel > D3D_HIGHEST_SHADER_MODEL) {
     const UINT Minor = (UINT)TestModel & 0x0f;
     LogCommentFmt(L"Installed SDK does not support "