[SPARK-38779][SQL][TESTS] Unify the pushed operator checking between …

…FileSource test suite and JDBCV2Suite

### What changes were proposed in this pull request?
In `JDBCV2Suite`, we use `checkPushedInfo` to check the pushed down operators. Will do the same for `FileSourceAggregatePushDownSuite`
```
  private def checkPushedInfo(df: DataFrame, expectedPlanFragment: String): Unit = {
    df.queryExecution.optimizedPlan.collect {
      case _: DataSourceV2ScanRelation =>
        checkKeywordsExistsInExplain(df, expectedPlanFragment)
    }
  }
```

### Why are the changes needed?
Make `FileSourceAggregatePushDownSuite` have the same way to check pushed down operators as `JDBCV2Suite` .

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Existing tests

Closes apache#36050 from huaxingao/minor.

Authored-by: huaxingao <huaxin_gao@apple.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>