[SPARK-37652][SQL]Add test for optimize skewed join through union

[mcdull-zhang]

What changes were proposed in this pull request?

#34974, solved most scenarios of data skew in union.
add test for it.

Why are the changes needed?

Added tests for the following scenarios：

scenes 1

Union
    SMJ
        ShuffleQueryStage
        ShuffleQueryStage
    SMJ
        ShuffleQueryStage
        ShuffleQueryStage

scenes 2

Union
    SMJ
        ShuffleQueryStage
        ShuffleQueryStage
    HashAggregate

scenes 3: not yet supported, SMJ-3 will introduce a new shuffle, so SMJ-1 cannot be optimized

Union
    SMJ-1
        ShuffleQueryStage
        ShuffleQueryStage
    SMJ-2
       SMJ-3
         ShuffleQueryStage
         ShuffleQueryStage
       HashAggregate

Does this PR introduce any user-facing change?

No

How was this patch tested?

Pass the added test

[AmplabJenkins]

Can one of the admins verify this patch?

[mcdull-zhang]

cc @cloud-fan

[zhengruifeng]

I think #34974 can handle this case.

[cloud-fan]

Yea, I think this is already supported.

[ulysses-you]

Although we have supported it, I think it's still good to add some test. @mcdull-zhang can you rebase this PR only for the test ?

[mcdull-zhang]

@ulysses-you @cloud-fan Please take a look, does the test code make sense? If it doesn't make sense, I'll turn off the pr.

[cloud-fan]

thanks, merging to master!