[SPARK-39514][CORE][TESTS] LauncherBackendSuite should add java.net.preferIPv6Addresses conf

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to make LauncherBackendSuite should use java.net.preferIPv6Addresses conf during launching.

Why are the changes needed?

BEFORE

$ SPARK_LOCAL_IP=::1 SBT_OPTS=-Djava.net.preferIPv6Addresses=true build/sbt "core/testOnly *.LauncherBackendSuite"
...
[info] LauncherBackendSuite:
[info] - local: launcher handle *** FAILED *** (30 seconds, 139 milliseconds)
[info]   The code passed to eventually never returned normally. Attempted 296 times over 30.078724458999996 seconds. Last failure message: The reference was null. (LauncherBackendSuite.scala:60)
...

AFTER

SPARK_LOCAL_IP=::1 SBT_OPTS=-Djava.net.preferIPv6Addresses=true build/sbt "core/testOnly *.LauncherBackendSuite"
...
[info] LauncherBackendSuite:
[info] - local: launcher handle (1 second, 432 milliseconds)
[info] - standalone/client: launcher handle (1 second, 600 milliseconds)
[info] Run completed in 3 seconds, 904 milliseconds.
[info] Total number of tests run: 2
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 2, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.

Does this PR introduce any user-facing change?

No, this is a test-only change.

How was this patch tested?

Pass the CIs and do the manual test.

[dongjoon-hyun]

Could you review this when you have some time, please, @wangyum ?

[wangyum]

Merged to master.

[dongjoon-hyun]

Thank you, @wangyum !