### What changes were proposed in this pull request?
This is a follow-up for SPARK-34346 which causes a flakiness due to `core-site.xml` test resource file addition. This PR aims to remove the test resource `core/src/test/resources/core-site.xml` from `core` module.
### Why are the changes needed?
Due to the test resource `core-site.xml`, YARN UT becomes flaky in GitHub Action and Jenkins.
```
$ build/sbt "yarn/testOnly *.YarnClusterSuite -- -z SPARK-16414" -Pyarn
...
[info] YarnClusterSuite:
[info] - yarn-cluster should respect conf overrides in SparkHadoopUtil (SPARK-16414, SPARK-23630) *** FAILED *** (20 seconds, 209 milliseconds)
[info] FAILED did not equal FINISHED (stdout/stderr was not captured) (BaseYarnClusterSuite.scala:210)
```
To isolate more, we may use `SPARK_TEST_HADOOP_CONF_DIR` like `yarn` module's `yarn/Client`, but it seems an overkill in `core` module.
```
// SPARK-23630: during testing, Spark scripts filter out hadoop conf dirs so that user's
// environments do not interfere with tests. This allows a special env variable during
// tests so that custom conf dirs can be used by unit tests.
val confDirs = Seq("HADOOP_CONF_DIR", "YARN_CONF_DIR") ++
(if (Utils.isTesting) Seq("SPARK_TEST_HADOOP_CONF_DIR") else Nil)
```
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Pass the CIs.
Closes#31515 from dongjoon-hyun/SPARK-34346-2.
Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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