[MINOR][SQL] Fix the test title from =!= to <=>, remove a duplicated test and add a test for =!=

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes three things as below:

• This test looks not testing <=> and identical with the test above, ===. So, it removes the test.

  -   test("<=>") {
  -     checkAnswer(
  -      testData2.filter($"a" === 1),
  -      testData2.collect().toSeq.filter(r => r.getInt(0) == 1))
  -
  -    checkAnswer(
  -      testData2.filter($"a" === $"b"),
  -      testData2.collect().toSeq.filter(r => r.getInt(0) == r.getInt(1)))
  -   }

• Replace the test title from =!= to <=>. It looks the test actually testing <=>.

  +  private lazy val nullData = Seq(
  +    (Some(1), Some(1)), (Some(1), Some(2)), (Some(1), None), (None, None)).toDF("a", "b")
  +
    ...
  -  test("=!=") {
  +  test("<=>") {
  -    val nullData = spark.createDataFrame(sparkContext.parallelize(
  -      Row(1, 1) ::
  -      Row(1, 2) ::
  -      Row(1, null) ::
  -      Row(null, null) :: Nil),
  -      StructType(Seq(StructField("a", IntegerType), StructField("b", IntegerType))))
  -
         checkAnswer(
           nullData.filter($"b" <=> 1),
    ...

• Add the tests for =!= which looks not existing.

  +  test("=!=") {
  +    checkAnswer(
  +      nullData.filter($"b" =!= 1),
  +      Row(1, 2) :: Nil)
  +
  +    checkAnswer(nullData.filter($"b" =!= null), Nil)
  +
  +    checkAnswer(
  +      nullData.filter($"a" =!= $"b"),
  +      Row(1, 2) :: Nil)
  +  }

How was this patch tested?

Manually running the tests.

[HyukjinKwon]

cc @rxin, it looks the last related change was made by you. Could you check if it makes sense please?

[SparkQA]

Test build #76413 has finished for PR 17842 at commit c164b88.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Merging in master/branch-2.2.