Improve racy subscribeToMore tests.

The use of setTimeout in tests is often an invitation to race conditions
and timing-sensitive outcomes.

This raciness became particularly problematic thanks to changes (between
Node 10 and Node 12) in the timing of Promise callbacks with respect to
setTimeout callbacks.

These changes also leave all tests passing when cherry-picked onto master,
so I'm confident I am *not* changing the tests just to fix my PR #6221, at
the expense of backwards compatibility.

src/__tests__/subscribeToMore.ts

@@ -72,14 +72,23 @@ describe('subscribeToMore', () => {
       link,
     });
 
-    const obsHandle = client.watchQuery<typeof req1['result']['data']>({
+    type TData = typeof req1['result']['data'];
+    const obsHandle = client.watchQuery<TData>({
       query,
     });
 
     const sub = obsHandle.subscribe({
-      next(queryResult) {
+      next(queryResult: TData) {
         latestResult = queryResult;
-        counter++;
+        if (++counter === 3) {
+          sub.unsubscribe();
+          expect(latestResult).toEqual({
+            data: { entry: { value: 'Amanda Liu' } },
+            loading: false,
+            networkStatus: 7,
+          });
+          resolve();
+        }
       },
     });
 
@@ -94,20 +103,15 @@ describe('subscribeToMore', () => {
       },
     });
 
-    setTimeout(() => {
-      sub.unsubscribe();
-      expect(counter).toBe(3);
-      expect(stripSymbols(latestResult)).toEqual({
-        data: { entry: { value: 'Amanda Liu' } },
-        loading: false,
-        networkStatus: 7,
-      });
-      resolve();
-    }, 15);
-
-    for (let i = 0; i < 2; i++) {
-      wSLink.simulateResult(results[i]);
+    let i = 0;
+    function simulate() {
+      const result = results[i++];
+      if (result) {
+        wSLink.simulateResult(result);
+        setTimeout(simulate, 10);
+      }
     }
+    simulate();
   });
 
   itAsync('calls error callback on error', (resolve, reject) => {
@@ -351,18 +355,25 @@ describe('subscribeToMore', () => {
       link,
     });
 
-    const obsHandle = client.watchQuery<
-      typeof typedReq['result']['data'],
-      typeof typedReq['request']['variables']
-    >({
+    type TData = typeof typedReq['result']['data'];
+    type TVars = typeof typedReq['request']['variables'];
+    const obsHandle = client.watchQuery<TData, TVars>({
       query,
       variables: { someNumber: 1 },
     });
 
     const sub = obsHandle.subscribe({
-      next(queryResult) {
+      next(queryResult: TData) {
         latestResult = queryResult;
-        counter++;
+        if (++counter === 3) {
+          sub.unsubscribe();
+          expect(latestResult).toEqual({
+            data: { entry: { value: 'Amanda Liu' } },
+            loading: false,
+            networkStatus: 7,
+          });
+          resolve();
+        }
       },
     });
 
@@ -380,19 +391,14 @@ describe('subscribeToMore', () => {
       },
     });
 
-    setTimeout(() => {
-      sub.unsubscribe();
-      expect(counter).toBe(3);
-      expect(stripSymbols(latestResult)).toEqual({
-        data: { entry: { value: 'Amanda Liu' } },
-        loading: false,
-        networkStatus: 7,
-      });
-      resolve();
-    }, 15);
-
-    for (let i = 0; i < 2; i++) {
-      wSLink.simulateResult(results[i]);
+    let i = 0;
+    function simulate() {
+      const result = results[i++];
+      if (result) {
+        wSLink.simulateResult(result);
+        setTimeout(simulate, 10);
+      }
     }
+    simulate();
   });
 });