fix: add global test fn for signals

Author: jhefferman-sfdc

packages/@lwc/shared/src/signals.ts

@@ -6,12 +6,19 @@
  */
 import { isFalse } from './assert';
 
-let trustedSignals: WeakSet<object>;
+let trustedSignals: WeakSet<object> | undefined;
 
 export function setTrustedSignalSet(signals: WeakSet<object>) {
     isFalse(trustedSignals, 'Trusted Signal Set is already set!');
 
     trustedSignals = signals;
+
+    // Only used in LWC's Karma. Contained within the set function as there are multiple imports of
+    // this module. Placing it here ensures we reference the import where the trustedSignals set is maintained
+    if (process.env.NODE_ENV === 'test-karma-lwc') {
+        // Used to reset the global state between test runs
+        (globalThis as any).__lwcResetTrustedSignalsSetForTest = () => (trustedSignals = undefined);
+    }
 }
 
 export function addTrustedSignal(signal: object) {