add test_devhub_support_form_throttle

[ioanarusiczki]

For testing mozilla/addons#16210, a dev hub form has been introduced.
https://addons.allizom.org/en-US/developers/support

The form has two stacked throttles:

• per-user-per-hour: 10 submissions
• per-IP-per-hour: 20 submissions

This test targets the per-user limit, which is the one that fires first when a single user submits the form repeatedly from a single IP.

Atm - in progress - passed locally in foreground.

[ioanarusiczki]

wfm on AMO stage - headless

but perhaps I'll leave this to be pushed after the requirements - updates See #1149