[SPARK-19050][SS][TESTS] Fix EventTimeWatermarkSuite 'delay in months and years handled correctly'

## What changes were proposed in this pull request?

`monthsSinceEpoch` in this test is like `math.floor(num)`, so `monthDiff` has two possible values.

## How was this patch tested?

Jenkins.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16449 from zsxwing/watermark-test-hotfix.

sql/core/src/test/scala/org/apache/spark/sql/streaming/EventTimeWatermarkSuite.scala

@@ -190,7 +190,10 @@ class EventTimeWatermarkSuite extends StreamTest with BeforeAndAfter with Loggin
       assertEventStats { e =>
         assert(timestampFormat.parse(e.get("max")).getTime === (currentTimeMs / 1000) * 1000)
         val watermarkTime = timestampFormat.parse(e.get("watermark"))
-        assert(monthsSinceEpoch(currentTime) - monthsSinceEpoch(watermarkTime) === 29)
+        val monthDiff = monthsSinceEpoch(currentTime) - monthsSinceEpoch(watermarkTime)
+        // monthsSinceEpoch is like `math.floor(num)`, so monthDiff has two possible values.
+        assert(monthDiff === 29 || monthDiff === 30,
+          s"currentTime: $currentTime, watermarkTime: $watermarkTime")
       }
     )
   }