[SPARK-32442][CORE][TESTS] Fix TaskSetManagerSuite by hiding o.a.s.FakeSchedulerBackend
### What changes were proposed in this pull request?
There exists two `FakeSchedulerBackend` classes.
```
$ git grep "class FakeSchedulerBackend"
core/src/test/scala/org/apache/spark/HeartbeatReceiverSuite.scala:private class FakeSchedulerBackend(
core/src/test/scala/org/apache/spark/scheduler/TaskSchedulerImplSuite.scala:class FakeSchedulerBackend extends SchedulerBackend {
```
This PR aims to hide the following at `TaskSetManagerSuite`.
```scala
import org.apache.spark.{FakeSchedulerBackend => _, _}
```
### Why are the changes needed?
Although `TaskSetManagerSuite` is inside `org.apache.spark.scheduler` package, `import org.apache.spark._` makes Scala 2.13 confused and causes 4 UT failures.
```
$ dev/change-scala-version.sh 2.13
$ build/mvn test -pl core --am -Pscala-2.13 -Dtest=none -DwildcardSuites=org.apache.spark.scheduler.TaskSetManagerSuite
...
Tests: succeeded 48, failed 4, canceled 0, ignored 0, pending 0
*** 4 TESTS FAILED ***
```
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
- **Scala 2.12**: Pass the Jenkins or GitHub Action
- **Scala 2.13**: Pass the following manually.
```
$ dev/change-scala-version.sh 2.13
$ build/mvn test -pl core --am -Pscala-2.13 -Dtest=none -DwildcardSuites=org.apache.spark.scheduler.TaskSetManagerSuite
...
Tests: succeeded 52, failed 0, canceled 0, ignored 0, pending 0
All tests passed.
```
Closes #29240 from dongjoon-hyun/SPARK-32442.
Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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