Fix a flaky test `DateTimeUtilsSuite.monthsBetween`

Error Message

3.949596773820191 did not equal 3.9495967741935485

Stacktrace

      org.scalatest.exceptions.TestFailedException: 3.949596773820191 did not equal 3.9495967741935485
      at org.scalatest.Assertions$class.newAssertionFailedException(Assertions.scala:528)
      at org.scalatest.FunSuite.newAssertionFailedException(FunSuite.scala:1560)
      at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:501)
      at org.apache.spark.sql.catalyst.util.DateTimeUtilsSuite$$anonfun$25.apply(DateTimeUtilsSuite.scala:495)
      at org.apache.spark.sql.catalyst.util.DateTimeUtilsSuite$$anonfun$25.apply(DateTimeUtilsSuite.scala:488)