[SPARK-57695][CORE] Make TestUtils.recursiveList null-safe by reusing Utils.recursiveList

### What changes were proposed in this pull request?

`TestUtils.recursiveList` duplicated the recursive directory walk already provided by `Utils.recursiveList` (i.e. `SparkFileUtils.recursiveList`). This PR removes the duplicated copy and delegates to `Utils.recursiveList`:

```scala
// before
def recursiveList(f: File): Array[File] = {
  require(f.isDirectory)
  val current = f.listFiles
  current ++ current.filter(_.isDirectory).flatMap(recursiveList)
}

// after
def recursiveList(f: File): Array[File] = Utils.recursiveList(f)
```

### Why are the changes needed?

The duplicated implementation carried the same two issues that were just fixed in `SparkFileUtils.recursiveList` under [SPARK-57530](https://issues.apache.org/jira/browse/SPARK-57530):

1. It called `File.listFiles` without a null check, so an IO error (or the directory being removed mid-walk) would throw an NPE.
2. The `current ++ ... .flatMap(...)` form had no linear-time guarantee.

By delegating to `Utils.recursiveList`, `TestUtils.recursiveList` automatically picks up the null-safety (a directory that cannot be listed is skipped with a warning instead of throwing) and the O(n) traversal from SPARK-57530, and the duplicated logic is removed.

This is a follow-up to SPARK-57530, which is already merged to master.

### Does this PR introduce _any_ user-facing change?

No. `TestUtils` is a test-only `private[spark]` helper; this is an internal refactor with no behavior change for any successful directory walk.

### How was this patch tested?

Existing tests that use `TestUtils.recursiveList` continue to exercise it. The behavioral contract for a readable directory tree is unchanged (same set of files returned); the only difference is that an unreadable directory is now skipped with a warning rather than throwing, matching `Utils.recursiveList`.

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

Generated-by: Claude Code

Closes #56901 from LuciferYang/SPARK-57695-testutils-recursivelist.

Authored-by: YangJie <yangjie01@baidu.com>
Signed-off-by: Max Gekk <max.gekk@gmail.com>
(cherry picked from commit a4ce62b)
Signed-off-by: Max Gekk <max.gekk@gmail.com>

Author: LuciferYang

core/src/main/scala/org/apache/spark/TestUtils.scala

@@ -387,11 +387,7 @@ private[spark] object TestUtils extends SparkTestUtils {
   /**
    * Lists files recursively.
    */
-  def recursiveList(f: File): Array[File] = {
-    require(f.isDirectory)
-    val current = f.listFiles
-    current ++ current.filter(_.isDirectory).flatMap(recursiveList)
-  }
+  def recursiveList(f: File): Array[File] = Utils.recursiveList(f)
 
   /**
    * Returns the list of files at 'path' recursively. This skips files that are ignored normally