fix demo function `convtest`: use test_signal instead of signal

9 jobs for better_convolutions in 9 minutes and 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Docker
passed #744681
build_docker_from_cache

00:00:18

 
  Test
passed #744683
test_mpi

00:05:11

passed #744682
test_serial

00:05:22

82.0%
 
  Demo Runs
passed #744688
run_bernoulli

00:00:38

passed #744689
run_curve_fitting

00:00:33

passed #744685
run_getting_started_1

00:00:38

passed #744686
run_getting_started_2

00:01:33

passed #744687
run_getting_started_3

00:03:30

passed #744684
run_ipynb

00:01:50