Include TestConfig.h only if DEFAULT_TEST_DIR is not defined (microsoft#4884)

TestConfig.h is not available in HLK test build.
This change enables skipping of the include.

Author: hekota

include/dxc/Test/HlslTestUtils.h

@@ -26,9 +26,12 @@
 #include "WEXAdapter.h"
 #endif
 #include "dxc/Support/Unicode.h"
-#include "dxc/Test/TestConfig.h"
 #include "dxc/DXIL/DxilConstants.h" // DenormMode
 
+#ifndef DEFAULT_TEST_DIR
+#include "dxc/Test/TestConfig.h"
+#endif
+
 using namespace std;
 
 #ifndef HLSLDATAFILEPARAM