Added intitial group leave tests

Fixed test readability, updated party test, and updated challenge update code when leaving group
Updated library, added group existance check, and reset full party
Updated syntax, added new userUnlinkChallenges, and added some initial testing for challenges
Added challenge tasks tests
Added try/catch to group remove, add more party tests, fixed broken challenge test, removed useless return value
Added public guild tests, added more tests to party, and abstracted remove invitations logic

Closes #6506

test/helpers/api-integration/v3/index.js

@@ -8,3 +8,11 @@ export { requester };
 export { translate } from '../translate';
 export { checkExistence, resetHabiticaDB } from '../mongo';
 export * from  './object-generators';
+
+export async function sleep (seconds) {
+  let milliseconds = seconds * 1000;
+
+  return new Promise((resolve) => {
+    setTimeout(resolve, milliseconds);
+  });
+}