add correct output directory for numabench tests

pkgs/default.nix

@@ -58,6 +58,10 @@ let
 
     numabench-check = numabench.overrideAttrs(old: {
       buildPhase = "make check";
+      installPhase =''
+        mkdir -p $out
+        cp -r ./tests $out/tests
+        '';
     });
     excit-check = excit.overrideAttrs(old: {
       buildPhase = "make check";