REGTESTS: add a few load balancing tests

REGTESTS: add a few load balancing tests

This adds "balance-rr" to test round robin, "balance-uri" to test the
default balance-uri method, and "balance-uri-path-only" which mixes H1
and H2 through "balance uri path-only" and verifies that they reach
the same server.

Note that for the latter, "proto h2" explicitly had to be placed on
the listening socket otherwise it would timeout. This may indicate an
issue in the H1->H2 upgrade depending how the H2 preface is sent maybe.