test: introduce method for testing variant of walkers

This commit adds a pattern that removes the strong dependency
on the compile-time DefaultWalkerType typename in generic unit tests.

Related-To: NEO-10641
Signed-off-by: Kamil Kopryk <kamil.kopryk@intel.com>

shared/source/gen9/hw_cmds_base.h

@@ -14,6 +14,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <variant>
 
 template <class T>
 struct CmdParse;
@@ -158,6 +159,8 @@ struct Gen9Family : public Gen9 {
     static constexpr bool isHeaplessMode() {
         return false;
     }
+
+    using WalkerVariant = std::variant<GPGPU_WALKER *>;
 };
 
 } // namespace NEO