test: fix flaky box/gh-4627-session-use-after-free

The problem was in that the test uses the global trigger
box.session.on_disconnect() to set a global variable by one
connection. But test-run can do multiple connects/reconnects to
the same instance. That led to multiple invocations of
box.session.on_disconnect(), which could override the global
variable in unexpected ways and moments.

The patch makes only one session execute that trigger.

Probably related to https://github.com/tarantool/test-run/issues/46


Follow up #4627

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>