fix windows shell error

Author: Lucccyo

src/sys/stm_tests.ml

@@ -147,7 +147,7 @@ struct
     | "Unix" -> ignore (Sys.command ("rm -rf " ^ (static_path / "sandbox_root") ^ " && mkdir " ^ (static_path / "sandbox_root")))
     | "Win32" ->
       ignore (Sys.command (
-      "powershell -Command \"Remove-Item -Path " ^ (static_path / "sandbox_root") ^ " -Recurse -Force\"
+      "powershell -Command \"Remove-Item -Path " ^ (static_path / "sandbox_root") ^ " -Recurse -Force -ErrorAction Ignore \"
        & mkdir " ^ (static_path / "sandbox_root")))
     | v -> failwith ("Sys tests not working with " ^ v)
 
@@ -159,7 +159,7 @@ struct
 
   let precond _c _s = true
 
-  let p path = (List.fold_left (/) (static_path / "sandbox_root") path)
+  let p path =  (List.fold_left (/) (static_path / "sandbox_root") path)
 
   let run c _file_name =
     match c with