Fix flaky tests

The test is flaky because on subscription, it is done on main thread but
asynchronously. Thus, if we don't drain the main queue, we could miss
the first update (while always get the final deleted update). This will
cause our fulfill logic fail.

This PR does the wait for main queue to drain.

Now, running `bazel test src/tests:Tests --runs_per_test=5000` will pass
without any failures (about half-an-hour to run on Mac Mini 2020).

src/tests/SubscribeTests.swift

@@ -398,6 +398,16 @@ class SubscribeTests: XCTestCase {
   ]
 
   #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+    func drainMainQueue() {
+      // Double dispatch to avoid nested main queue dispatch in previous blocks.
+      let mainQueueDrain = XCTestExpectation(description: "main")
+      DispatchQueue.main.async {
+        DispatchQueue.main.async {
+          mainQueueDrain.fulfill()
+        }
+      }
+      wait(for: [mainQueueDrain], timeout: 10.0)
+    }
     @available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
     func testObjectPublisher() {
       guard let dflat = dflat else { return }
@@ -432,6 +442,9 @@ class SubscribeTests: XCTestCase {
             pubExpectation.fulfill()
           }
         }
+      // The subscription happens on main queue asynchronously. Drain it to avoid we skip
+      // directly to deletion.
+      drainMainQueue()
       let firstExpectation = XCTestExpectation(description: "transcation done")
       dflat.performChanges(
         [MyGame.Sample.Monster.self],
@@ -494,6 +507,9 @@ class SubscribeTests: XCTestCase {
             subExpectation.fulfill()
           }
         }
+      // The subscription happens on main queue asynchronously. Drain it to avoid we skip
+      // directly to deletion.
+      drainMainQueue()
       // Add one.
       let firstExpectation = XCTestExpectation(description: "transcation done")
       dflat.performChanges(