Fix alluxio-fuse mount reporting success when the mount failed

[Stephen-Pu]

Fix alluxio-fuse mount (integration/fuse/bin/alluxio-fuse) reporting a
successful mount when the mount actually failed.

Previously the mount outcome was inferred from whether a background process was
still alive after a fixed sleep, and $! referred to the wrapping subshell
( nohup ... ) & rather than the AlluxioFuse process itself:

(nohup ${cmd} > ${ALLUXIO_LOGS_DIR}/fuse.out 2>&1) &
sleep ${mount_sleep_seconds}
if kill -0 $! > /dev/null 2>&1 ; then   # "success" == some pid alive after 2s

This PR:

• Drops the subshell wrapper so $! is the real AlluxioFuse pid.
• Adds a wait_for_fuse_mount(pid, timeout) helper that polls up to the
  configured timeout and reports success only once the mount point actually
  appears in the mount table (reusing the existing fuse_mounted helper),
  bailing out early if the process exits first.

The existing -t option (mount_sleep_seconds) is reused as the poll budget,
so there is no new configuration.

Fixes #15220. A process-liveness check alone cannot distinguish "mount
succeeded" from "process is still starting up / slow to fail but the mount never
happened", so an invalid mount (e.g. alluxio-fuse mount / <path>) was reported
as Successfully mounted even though it failed. Verifying that the mount point
is actually present makes the success/failure report correct.

The alluxio-fuse mount command now reports Failed to mount ... (exit code 1)
when the mount does not actually establish, instead of incorrectly reporting
success. No API or property-key changes.

[alluxio-bot]

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[apc999]

alluxio-bot, merge this please

[alluxio-bot]

merge failed:
Merge refused because the PR has the label cla-missing