Make tests wait for syncs to happen

Add lots of calls to `syncPromise` to cope with the fact that sync responses
are now handled asynchronously, which makes them prone to races otherwise.

Also a quick sanity-check in crypto to make one of the test failures less
cryptic.

spec/TestClient.js

@@ -77,7 +77,10 @@ TestClient.prototype.start = function() {
         pendingEventOrdering: 'detached',
     });
 
-    return this.httpBackend.flushAllExpected().then(() => {
+    return Promise.all([
+        this.httpBackend.flushAllExpected(),
+        testUtils.syncPromise(this.client),
+    ]).then(() => {
         console.log(this + ': started');
     });
 };
@@ -199,5 +202,7 @@ TestClient.prototype.flushSync = function() {
     return Promise.all([
         this.httpBackend.flush('/sync', 1),
         testUtils.syncPromise(this.client),
-    ]);
+    ]).then(() => {
+        console.log(`${this}: flushSync completed`);
+    });
 };