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

## 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.

  ```diff
  -   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 `<=>`.

  ```diff
  +  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.

  ```diff
  +  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.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17842 from HyukjinKwon/minor-test-fix.