We used to set it to ${h1_px_addr} but it randomly fails on certain
hosts (FreeBSD and OSX) where the address is surprisingly set to "::1"
while the Host field contains 127.0.0.1 (hence two different address
families). While this is likely a minor issue in vtest, we don't need
to depend on this and can easily hard-code 127.0.0.1 which is already
used in other 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.
