Listing microseconds in bridgestrap status output is confusing

(This ticket is just a simple UX improvement, but hopefully still a useful one :) I've tagged it as 'First Contribution' since it's a good opportunity for somebody to get some experience making a git commit etc.)

Compare the current output format:

* obfs4: dysfunctional
  Error: timed out waiting for bridge descriptor
  Last tested: 2021-01-17 10:38:22.671859857 +0000 UTC (8h41m42.941665234s ago)

to this simpler alternative:

* obfs4: dysfunctional
  Error: timed out waiting for bridge descriptor
  Last tested: 2021-01-17 10:38:22 +0000 UTC (8h41m42s ago)

At present, having so much precision in the fraction of the seconds draws the reader's eye to that number, and that number is the least important part of the output.

Thanks!