HS: clear _hs_null_error at test boundary — fix bootstrap/can wait (+1 test)

hs-win-call sets window._hs_null_error as a side channel when a global
function lookup fails. _driveAsync checks this flag and bails early to
avoid error cascades, but the flag was never cleared between tests.

A previous test (call/can call functions w/ underscores) triggers
hs-win-call when global_function is not set up, which leaves
_hs_null_error="'global_function' is null". The bootstrap/can wait test
then calls `wait 20ms` whose io-sleep resume is skipped by _driveAsync,
so .bar is never added and the assertion fails.

Fix: clear _hs_null_error in the per-test reset block in the test runner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/hs-run-filtered.js

@@ -809,6 +809,7 @@ for(let i=startTest;i<Math.min(endTest,testCount);i++){
 
   // Reset body
   _body.children=[];_body.childNodes=[];_body.innerHTML='';_body.textContent='';
+  globalThis._hs_null_error=null;
   globalThis.__test_selection='';
   globalThis.__hsCookieStore.clear();
   globalThis.__hsMutationRegistry.length = 0;