[tunnelbroker] Switch Tunnelbroker unit tests to autoscaling CI runners

Summary:
Pretty much identical to D10328 and D10339, but for Tunnelbroker.

Context: https://linear.app/comm/issue/ENG-5770/office-mba-runner-out-of-storage#comment-d61c38e9

Test Plan:
See if CI passes

Reviewers: ashoat, ginsu, tomek, rohan, bartek, varun

Reviewed By: bartek

Differential Revision: https://phab.comm.dev/D10340