test_try_again_many_times(): use three times `None`'s ref count

100,000 iterations was arbitrary anyway, so let's just depend on the
initial reference count of `None` and iterate three times that since
this is the object we want to avoid destroy.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3d587c730b4587d5381d07541d0d86ccff344858
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1822
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>