bridgetest: make output format more robust

Here's the preliminary data format for the logs that come out of the bridgetest docker image:
https://gitlab.torproject.org/cohosh/probetest#data-format

and there are some scripts like
https://gitlab.torproject.org/cohosh/probetest/-/blob/main/analysis/makecsv
that help to generate the csv format.

We should look through peer formats, like OONI's data format and censored planet, and get ideas for what else we'll wish we'd included.

For examples:

• timestamp of when the test happened (perhaps rounded to something granular)
• which version of the test generated this entry
• more detail about the test target, e.g. the hashed fingerprint of the bridge that we tested.
• time spent from start of test until result (I realized this one after looking at the various aborted tests from China, where if we knew how long the successful tests took, and found that they were already bumping up against our timeout, that would be useful to know.)
• (maybe, I'm not sure, we should ask others for advice) All of the stuff that's implicit in the filename and directory structure, so it's in the file too -- which country we're testing from, AS we map to, the name of the test, etc.