[SPARK-21006][TESTS] Create rpcEnv and run later needs shutdown and awaitTermination

Signed-off-by: 10087686 <wang.jiaochunzte.com.cn>

## What changes were proposed in this pull request?
When  run test("port conflict") case, we need run anotherEnv.shutdown() and anotherEnv.awaitTermination() for free resource.
(Please fill in changes proposed in this fix)

## How was this patch tested?
run RpcEnvSuit.scala Utest
(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: 10087686 <wang.jiaochun@zte.com.cn>

Closes #18226 from wangjiaochun/master.