[SPARK-51630][CORE][TESTS] Remove pids size check from "SPARK-45907: Use ProcessHandle APIs to computeProcessTree in ProcfsMetricsGetter"

### What changes were proposed in this pull request?
This PR removes the size check for `pids` from the test case titled "SPARK-45907: Use ProcessHandle APIs to computeProcessTree in ProcfsMetricsGetter".

### Why are the changes needed?
To avoid potential test instability, the test case 'SPARK-45907: Use ProcessHandle APIs to computeProcessTree in ProcfsMetricsGetter' may fail when tested in the following environment:

```
Apple M3
macOS 15.4
zulu 17.0.14
```

run `build/sbt "core/testOnly org.apache.spark.ui.UISeleniumSuite org.apache.spark.executor.ProcfsMetricsGetterSuite"`

```
[info] UISeleniumSuite:
[info] - all jobs page should be rendered even though we configure the scheduling mode to fair (4 seconds, 202 milliseconds)
[info] - effects of unpersist() / persist() should be reflected (2 seconds, 845 milliseconds)
[info] - failed stages should not appear to be active (2 seconds, 455 milliseconds)
[info] - spark.ui.killEnabled should properly control kill button display (8 seconds, 610 milliseconds)
[info] - jobs page should not display job group name unless some job was submitted in a job group (2 seconds, 546 milliseconds)
[info] - job progress bars should handle stage / task failures (2 seconds, 610 milliseconds)
[info] - job details page should display useful information for stages that haven't started (2 seconds, 292 milliseconds)
[info] - job progress bars / cells reflect skipped stages / tasks (2 seconds, 304 milliseconds)
[info] - stages that aren't run appear as 'skipped stages' after a job finishes (2 seconds, 201 milliseconds)
[info] - jobs with stages that are skipped should show correct link descriptions on all jobs page (2 seconds, 188 milliseconds)
[info] - attaching and detaching a new tab (2 seconds, 268 milliseconds)
[info] - kill stage POST/GET response is correct (173 milliseconds)
[info] - kill job POST/GET response is correct (141 milliseconds)
[info] - stage & job retention (2 seconds, 661 milliseconds)
[info] - live UI json application list (2 seconds, 187 milliseconds)
[info] - job stages should have expected dotfile under DAG visualization (2 seconds, 126 milliseconds)
[info] - stages page should show skipped stages (2 seconds, 651 milliseconds)
[info] - Staleness of Spark UI should not last minutes or hours (2 seconds, 167 milliseconds)
[info] - description for empty jobs (2 seconds, 242 milliseconds)
[info] - Support disable event timeline (4 seconds, 585 milliseconds)
[info] - SPARK-41365: Stage page can be accessed if URI was encoded twice (2 seconds, 306 milliseconds)
[info] - SPARK-44895: Add 'daemon', 'priority' for ThreadStackTrace (2 seconds, 219 milliseconds)
[info] ProcfsMetricsGetterSuite:
[info] - testGetProcessInfo (1 millisecond)
OpenJDK 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended
[info] - SPARK-34845: partial metrics shouldn't be returned (493 milliseconds)
[info] - SPARK-45907: Use ProcessHandle APIs to computeProcessTree in ProcfsMetricsGetter *** FAILED *** (10 seconds, 149 milliseconds)
[info]   The code passed to eventually never returned normally. Attempted 102 times over 10.036665625 seconds. Last failure message: 1 did not equal 3. (ProcfsMetricsGetterSuite.scala:87)
[info]   org.scalatest.exceptions.TestFailedDueToTimeoutException:
[info]   at org.scalatest.enablers.Retrying$$anon$4.tryTryAgain$2(Retrying.scala:219)
[info]   at org.scalatest.enablers.Retrying$$anon$4.retry(Retrying.scala:226)
[info]   at org.scalatest.concurrent.Eventually.eventually(Eventually.scala:313)
[info]   at org.scalatest.concurrent.Eventually.eventually$(Eventually.scala:312)
[info]   at org.scalatest.concurrent.Eventually$.eventually(Eventually.scala:457)
[info]   at org.apache.spark.executor.ProcfsMetricsGetterSuite.$anonfun$new$3(ProcfsMetricsGetterSuite.scala:87)
```

After conducting an investigation, I discovered that the `eventually` block does not always capture the stage where `pids.size` is 3. Due to timing issues, it may directly capture the scenario where `pids.size` is 4. Furthermore, since the checks for `pids.contains(currentPid)` and `pids.contains(child)` are more crucial, this PR removes the check for the size of `pids`.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
- Pass GitHub Actions

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #50545 from LuciferYang/ProcfsMetricsGetterSuite.

Lead-authored-by: yangjie01 <yangjie01@baidu.com>
Co-authored-by: YangJie <yangjie01@baidu.com>
Signed-off-by: yangjie01 <yangjie01@baidu.com>

Author: LuciferYang

core/src/test/scala/org/apache/spark/executor/ProcfsMetricsGetterSuite.scala

@@ -86,7 +86,6 @@ class ProcfsMetricsGetterSuite extends SparkFunSuite {
       val child = process.toHandle.pid()
       eventually(timeout(10.seconds), interval(100.milliseconds)) {
         val pids = p.computeProcessTree()
-        assert(pids.size === 3)
         assert(pids.contains(currentPid))
         assert(pids.contains(child))
       }