How do you keep your phone vibrating indefinitely?
Since it is a one time test, suggest a simple macro using Macrodroid

Use + to add Trigger /actions from the corresponding panel

Trigger: Empty Trigger

Actions


Repeat actions &rarr; Fixed number &rarr; OK &rarr; Fixed count say 70
Tap on end loop &rarr; Add action above &rarr; Vibrate &rarr; Long buzz &rarr; OK
Tap on end loop again to add one more action of wait 1 second


From Overflow menu choose Test Macro and your phone should buzz almost continuously for long enough ( increase count in step 1 if required )