### What changes were proposed in this pull request?
1, remove newly added method: `def testChiSquare(dataset: Dataset[_], featuresCol: String, labelCol: String): Array[SelectionTestResult]`, because: 1) it is only used in `ChiSqSelector`; 2, since the returned dataframe of `def test(dataset: DataFrame, featuresCol: String, labelCol: String): DataFrame` only contains one row, after collect it back to driver, the results are similar;
2, add method `def test(dataset: DataFrame, featuresCol: String, labelCol: String, flatten: Boolean): DataFrame` to return the flatten results;
### Why are the changes needed?
1, when get returned result dataframe, we may want to filter it like `pValue<0.1`, but current returned dataframe is hard to use;
2, current impl need to collect all test results of all columns back to the driver, this is a bottleneck, if we return the flatten datafame, we no longer to collect them to driver;
### Does this PR introduce any user-facing change?
Yes:
1, `def testChiSquare(dataset: Dataset[_], featuresCol: String, labelCol: String): Array[SelectionTestResult]` removed;
2, the returned dataframe need an action to trigger computation;
### How was this patch tested?
updated testsuites
Closes#28176 from zhengruifeng/flatten_chisq.
Authored-by: zhengruifeng <ruifengz@foxmail.com>
Signed-off-by: zhengruifeng <ruifengz@foxmail.com>
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