tests: bsim: bluetooth: mesh: Change error code on timeout

This commit changes error code returned by `k_sem_take` when it times
out from `-EAGAIN` to `-ETIMEDOUT` because `bt_le_scan_start` also
returns `-EAGAIN` if bt device is not ready, which may lead to invalid
test behavior.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>