[SPARK-18338][SQL][TEST-MAVEN] Fix test case initialization order under Maven builds

## What changes were proposed in this pull request?

Test case initialization order under Maven and SBT are different. Maven always creates instances of all test cases and then run them all together.

This fails `ObjectHashAggregateSuite` because the randomized test cases there register a temporary Hive function right before creating a test case, and can be cleared while initializing other successive test cases. In SBT, this is fine since the created test case is executed immediately after creating the temporary function.

To fix this issue, we should put initialization/destruction code into `beforeAll()` and `afterAll()`.

## How was this patch tested?

Existing tests.

Author: Cheng Lian <lian@databricks.com>

Closes #15802 from liancheng/fix-flaky-object-hash-agg-suite.