tests: move classes around in lttng_live_server.py

tests: move classes around in lttng_live_server.py

In preparation for an upcoming patch, which fixes typing issues in
lttng_live_server.py, move some types around to avoid forward-references
when possible.

Change-Id: If7c14db977fa8b5c5d050badf2d8f1254fb1279a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10870
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>