## What changes were proposed in this pull request?
Refactor ObjectHashAggregateExecBenchmark to use main method
## How was this patch tested?
Manually tested:
```
bin/spark-submit --class org.apache.spark.sql.execution.benchmark.ObjectHashAggregateExecBenchmark --jars sql/catalyst/target/spark-catalyst_2.11-3.0.0-SNAPSHOT-tests.jar,core/target/spark-core_2.11-3.0.0-SNAPSHOT-tests.jar,sql/hive/target/spark-hive_2.11-3.0.0-SNAPSHOT.jar --packages org.spark-project.hive:hive-exec:1.2.1.spark2 sql/hive/target/spark-hive_2.11-3.0.0-SNAPSHOT-tests.jar
```
Generated results with:
```
SPARK_GENERATE_BENCHMARK_FILES=1 build/sbt "hive/test:runMain org.apache.spark.sql.execution.benchmark.ObjectHashAggregateExecBenchmark"
```
Closes#22804 from peter-toth/SPARK-25665.
Lead-authored-by: Peter Toth <peter.toth@gmail.com>
Co-authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
ccd07b7366