eng: move selfhost test provider as a workspace extension (#208699)

Testing #208184, closes #207756

.vscode/extensions/vscode-selfhost-test-provider/src/memoize.ts

@@ -0,0 +1,16 @@
+/*---------------------------------------------------------
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------*/
+
+export const memoizeLast = <A, T>(fn: (args: A) => T): ((args: A) => T) => {
+  let last: { arg: A; result: T } | undefined;
+  return arg => {
+    if (last && last.arg === arg) {
+      return last.result;
+    }
+
+    const result = fn(arg);
+    last = { arg, result };
+    return result;
+  };
+};