msglist diffing tests: Fix a flake with stream-subscribed status chan…

…ging

In 371e9b4, we said we preferred spreading
eg.plusReduxState.subscriptions...but I guess at that point we
didn't predict this flake.

See 932c28d for a solution to another flake that involved streams
unstably being subscribed / unsubscribed.

Fixes: zulip#5414

src/webview/__tests__/generateInboundEventEditSequence-test.js

@@ -198,7 +198,9 @@ const pmMessages6 = [
 const baseState = eg.reduxStatePlus({
   streams: [...eg.plusReduxState.streams, stream1, stream2],
   subscriptions: [
-    ...eg.plusReduxState.subscriptions,
+    // Could spread eg.plusReduxState.subscriptions, for completeness…but
+    // then we'd invite flakes where random stream IDs in there sometimes
+    // collide with our fixed ones, changing whether a stream is subscribed
     eg.makeSubscription({ stream: stream1 }),
     eg.makeSubscription({ stream: stream2 }),
   ],
@@ -437,8 +439,12 @@ describe('messages -> piece descriptors -> content HTML is stable/sensible', ()
         state: eg.reduxStatePlus({
           streams: [...eg.plusReduxState.streams, stream1, stream2],
           subscriptions: [
-            ...eg.plusReduxState.subscriptions,
             // no subscription for stream1
+            //
+            // Could spread eg.plusReduxState.subscriptions, for
+            // completeness…but then we'd invite flakes where random stream
+            // IDs in there sometimes collide with our fixed ones, changing
+            // whether a stream is subscribed
             eg.makeSubscription({ stream: stream2 }),
           ],
         }),