REGTESTS: sample_fetches: test the stream state fetches over HTTP/2

REGTESTS: sample_fetches: test the stream state fetches over HTTP/2

fs.id was only covered by a QUIC test, and bs.id, fs.aborted,
bs.aborted, fs.rst_code, bs.rst_code, fc_settings_streams_limit,
bc_settings_streams_limit and bc_glitches had no coverage at all. They
describe the multiplexed stream and its connection, so they need HTTP/2
on both sides, which vtest provides.

The first listener runs two streams over the same pair of connections,
which shows the stream ids of both sides and the two stream limits, the
frontend one being what haproxy announces and the backend one what the
server announced. The second listener has its stream reset by the
client and the third one by the server, and since no response reaches
the client in those two cases, the abort flag and the reset code are
read from the log.