The really lo-fi solution, in a Terminal:
while true; do /Applications/Speedtest.app && sleep 1800; done
But you can do it without typing in Automator. Create a new Application, then:
– Launch Application (pick Speedtest)
– Pause (for 30 minutes)
– Loop (select loop automatically, and you need to specify a timeout to avoid infinite loops, so put something like 1000 times)
Press Run, and it will start Speedtest every 30 minutes. This assumes that speedtest does a test when it starts. You might need to put a Quit Application before Launch if it only does a test when it is launched, otherwise you’ll have to quit it by hand.