Rewrite to take test2json input

[tbg]

This executes on the plan outlined in:

https://github.com/cockroachdb/go-test-teamcity/issues/5#issuecomment-422494665

by making go-test-teamcity take test2json input. More precisely, and
unfortunately, it realistically has to be taking the input of

make test TESTFLAGS='-v -json'

which isn't pure JSON but contains a bunch of make and go build
output (requiring a bit of massaging to pluck the relevant JSON from
the input stream).

I retained the old functionality and introduced a new -json flag
that triggers the new code. This important because our CI scripts
are versioned, and so we need the updated code here to work with
the non-updated CI pipeline.

I initially tried to jsonify the old test cases, but they're basically
garbage and don't correspond to real test output in some cases. Instead,
I generated a new corpus from actual CI runs in the main repo from which
I copied the artifacts file:

cockroachdb/cockroach#30406

I think it's worth checking in that PR in suitable form because it'll
allow us to iterate on this parser and extend the corpus much more
easily in the future.

Two caveats:

1. the code isn't creating TeamCity Test Suite tags. I was never married
   to those and have found it unfortunate that they mess up TeamCity's test
   duration counting (as far as I can remember). Adding them is probably
   possible, but with parallel tests (which we don't use heavily) and such
   I'm pretty sure that any naive implementation would be incorrect.
2. Tests which aren't explicitly ended (due to premature exit/crash of
   the package test binary) are now output at the very end of the log.
   Since the input contains all tests, this can be a little unintuitive.
   However, these final tests contain basically no information anyway, so
   that seems fine. This could be improved by flushing out the tests
   whenever a package-level PASS/FAIL event is encountered, but I'm not
   sure it's worth doing so.

The next step, assuming the code is in good order, is to merge it and
update the CI pipeline in the main repo to pass the -json flags both
to go test and go-test-teamcity.

[benesch]

I only skimmed this, but the part where we can conditionally turn this on in an upstream PR by passing -json seems great. Don't think you need to wait for a more thorough review.

[tbg]

TFTR!