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.

(cherry picked from commit 19490907b3836369215a9860f1bd6d2bc3297937)
[wt: marked the balance-uri-path-only for 2.2 since already backported]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 6a551a0ea58218ab02f45b0fb1eb26248b7a5263)
[wt: marked the balance-uri-path-only for 2.1 since already backported]
Signed-off-by: Willy Tarreau <w@1wt.eu>