internal_link test: Push store setup properly inside test code

Generally the application code should be run inside test cases, and
not from the test-discovery code that runs before the test harness
even knows what the list of test cases is.  That means it should be
called from inside a `test` callback, not from `main` itself or from
a `group` callback.

There's a lot of application code that runs as part of calls like
`store.addStreams` and `store.addUser`.  So this `setupStore` method
should be called from inside the test cases.  Do that.

In particular this is needed in order to make an async method out of
PerAccountStore.handleEvent, which underlies those helpers like
`addStreams`.  Async calls especially need to be kept out of
test-discovery time, because `package:test` requires all the test
discovery to be completed synchronously.

Author: gnprice

test/model/internal_link_test.dart

@@ -31,17 +31,16 @@ void main() {
 
   void testExpectedNarrows(List<(String, Narrow?)> testCases, {
     List<ZulipStream>? streams,
-    PerAccountStore? store,
     List<User>? users,
   }) {
-    assert((streams != null || users != null) ^ (store != null));
-    store ??= setupStore(realmUrl: realmUrl, streams: streams, users: users);
+    assert(streams != null || users != null);
     for (final testCase in testCases) {
       final String urlString = testCase.$1;
-      final Uri url = store.tryResolveUrl(urlString)!;
       final Narrow? expected = testCase.$2;
       test(urlString, () {
-        check(parseInternalLink(url, store!)).equals(expected);
+        final store = setupStore(realmUrl: realmUrl, streams: streams, users: users);
+        final url = store.tryResolveUrl(urlString)!;
+        check(parseInternalLink(url, store)).equals(expected);
       });
     }
   }