Add additional checks for three existing unit tests (#8973)

Summary:
With test sync points, we can assert on the equality of iterator value in three existing
unit tests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8973

Test Plan:
```
gtest-parallel -r 1000 ./db_test2 --gtest_filter=DBTest2.IterRaceFlush2:DBTest2.IterRaceFlush1:DBTest2.IterRefreshRaceFlush
```

make check

Reviewed By: akankshamahajan15

Differential Revision: D31256340

Pulled By: riversand963

fbshipit-source-id: a9440767ab383e0ec61bd43ffa8fbec4ba562ea2

db/db_test2.cc

@@ -3670,14 +3670,15 @@ TEST_F(DBTest2, IterRaceFlush1) {
     TEST_SYNC_POINT("DBTest2::IterRaceFlush:2");
   });
 
-  // iterator is created after the first Put(), so it should see either
-  // "v1" or "v2".
+  // iterator is created after the first Put(), and its snapshot sequence is
+  // assigned after second Put(), so it must see v2.
   {
     std::unique_ptr<Iterator> it(db_->NewIterator(ReadOptions()));
     it->Seek("foo");
     ASSERT_TRUE(it->Valid());
     ASSERT_OK(it->status());
     ASSERT_EQ("foo", it->key().ToString());
+    ASSERT_EQ("v2", it->value().ToString());
   }
 
   t1.join();
@@ -3700,14 +3701,15 @@ TEST_F(DBTest2, IterRaceFlush2) {
     TEST_SYNC_POINT("DBTest2::IterRaceFlush2:2");
   });
 
-  // iterator is created after the first Put(), so it should see either
-  // "v1" or "v2".
+  // iterator is created after the first Put(), and its snapshot sequence is
+  // assigned before second Put(), thus it must see v1.
   {
     std::unique_ptr<Iterator> it(db_->NewIterator(ReadOptions()));
     it->Seek("foo");
     ASSERT_TRUE(it->Valid());
     ASSERT_OK(it->status());
     ASSERT_EQ("foo", it->key().ToString());
+    ASSERT_EQ("v1", it->value().ToString());
   }
 
   t1.join();
@@ -3730,8 +3732,8 @@ TEST_F(DBTest2, IterRefreshRaceFlush) {
     TEST_SYNC_POINT("DBTest2::IterRefreshRaceFlush:2");
   });
 
-  // iterator is created after the first Put(), so it should see either
-  // "v1" or "v2".
+  // iterator is refreshed after the first Put(), and its sequence number is
+  // assigned after second Put(), thus it must see v2.
   {
     std::unique_ptr<Iterator> it(db_->NewIterator(ReadOptions()));
     ASSERT_OK(it->status());
@@ -3740,6 +3742,7 @@ TEST_F(DBTest2, IterRefreshRaceFlush) {
     ASSERT_TRUE(it->Valid());
     ASSERT_OK(it->status());
     ASSERT_EQ("foo", it->key().ToString());
+    ASSERT_EQ("v2", it->value().ToString());
   }
 
   t1.join();