tests: net: trickle: use separate semaphore for second trickle timer

A second semaphore is used for the second trickle timer, so that if the
first timer expires twice before the second one, the test would still
wait before proceeding to check on cb_2_called.

Fixes #18598.

Backported to v1.14 in order to fix PR #24300
Fixes #24300

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>