Do not include TestConfig.h for all HLK build (microsoft#4887)

TestConfig.h is not available in HLK test build.
The test library uses _HLK_CONF define to distinguish
between Exec tests-only and HLK-only code.

Author: hekota

include/dxc/Test/HlslTestUtils.h

@@ -28,7 +28,9 @@
 #include "dxc/Support/Unicode.h"
 #include "dxc/DXIL/DxilConstants.h" // DenormMode
 
-#ifndef DEFAULT_TEST_DIR
+#ifdef _HLK_CONF
+#define DEFAULT_TEST_DIR ""
+#else
 #include "dxc/Test/TestConfig.h"
 #endif