test_runner: add __proto__: null to extraOverrides object literal

Node.js core lint rule (node-core/set-proto-to-null-in-object)
requires every object literal in lib/ to have __proto__: null
to prevent prototype pollution.

Author: Felipeness

lib/internal/test_runner/test.js

@@ -288,6 +288,7 @@ class TestContext {
       (keyword) => {
         this.test[keyword] = (name, options, fn) =>
           runSubtest(name, options, fn, getCallerLocation(), {
+            __proto__: null,
             [keyword]: true,
           });
       },