[SPARK-21524][ML] unit test fix: ValidatorParamsSuiteHelpers generates wrong temp files

## What changes were proposed in this pull request?
jira: https://issues.apache.org/jira/browse/SPARK-21524

ValidatorParamsSuiteHelpers.testFileMove() is generating temp dir in the wrong place and does not delete them.

ValidatorParamsSuiteHelpers.testFileMove() is invoked by TrainValidationSplitSuite and crossValidatorSuite. Currently it uses `tempDir` from `TempDirectory`, which unfortunately is never initialized since the `boforeAll()` of `ValidatorParamsSuiteHelpers` is never invoked.

In my system, it leaves some temp directories in the assembly folder each time I run the TrainValidationSplitSuite and crossValidatorSuite.

## How was this patch tested?
unit test fix

Author: Yuhao Yang <yuhao.yang@intel.com>

Closes #18728 from hhbyyh/tempDirFix.