Disallow mocha context updates within test functions

Fixed: 422717761
Change-Id: I72bdec93fa0ba8b272107c5a0bf8a332347f7b40
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6624176
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>

Author: OrKoN

test/e2e_non_hosted/conductor/mocha-interface-helpers.ts

@@ -87,7 +87,8 @@ export function makeInstrumentedTestFunction(fn: Mocha.AsyncFunc, label: string,
         debugger;  // If you're paused here while debugging, stepping into the next line will step into your test.
       }
       const testResult =
-          await (state === undefined ? fn.call(this) : (fn as E2E.TestAsyncCallbackWithState).call(this, state.state));
+          await (state === undefined ? fn.call(this) :
+                                       (fn as unknown as E2E.TestAsyncCallbackWithState).call(undefined, state.state));
       dumpCollectedErrors();
       return testResult;
     })();

test/e2e_non_hosted/types.d.ts

@@ -53,6 +53,7 @@ declare global {
       browser: BrowserWrapper;
     }
 
-    export type TestAsyncCallbackWithState = (this: Mocha.Context, state: State) => PromiseLike<unknown>;
+    // We do not allow test functions to affect mocha context.
+    export type TestAsyncCallbackWithState = (this: undefined, state: State) => PromiseLike<unknown>;
   }
 }