tracing: selftests: Extend hotplug testing for trace remotes

tracing: selftests: Extend hotplug testing for trace remotes

The hotplug testing only tries reading a trace remote buffer, loaded
before a CPU is offline. Extend this testing to cover:

  * A trace remote buffer loaded after a CPU is offline.
  * A trace remote buffer loaded before a CPU is online.

Because of these added test cases, move the hotplug testing into a
separate hotplug.tc file.

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://patch.msgid.link/20260401045100.3394299-3-vdonnefort@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>