[SPARK-53179][CORE][TESTS] Use SparkStreamUtils.toString instead of CharStreams.toString

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to se SparkStreamUtils.toString instead of CharStreams.toString.

Why are the changes needed?

SparkStreamUtils.toString is faster than CharStreams.toString.

scala> spark.time(org.apache.spark.util.SparkStreamUtils.toString(new java.io.FileInputStream("/tmp/1G.bin")).length)
Time taken: 322 ms
val res0: Int = 1073741824

scala> spark.time(com.google.common.io.CharStreams.toString(new java.io.InputStreamReader(java.nio.file.Files.newInputStream(Path.of("/tmp/1G.bin")))).length)
Time taken: 533 ms
val res1: Int = 1073741824

Does this PR introduce any user-facing change?

No behavior change.

How was this patch tested?

Pass the CIs.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Could you review this test PR, @peter-toth ?

[dongjoon-hyun]

Thank you always, @peter-toth !

[dongjoon-hyun]

Merged to master for Apache Spark 4.1.0.