REGTESTS: proxy: test the "option checkcache" response filter

REGTESTS: proxy: test the "option checkcache" response filter

"option checkcache" had no coverage. The test walks through the
conditions listed in the documentation: a cacheable 200 carrying a
Set-Cookie is blocked, a response made non cacheable by "cache-control:
private", "no-store", "no-cache=\"set-cookie\"" or "pragma: no-cache"
goes through, a status which is not cacheable by default goes through
unless the server declares the response public, and the response to a
POST is not filtered.

The origin is a second haproxy instance crafting one response per path,
because a blocked response makes the proxy close the server connection,
which a single vtest server session would not survive.

The test also documents one behavior which the documentation does not
mention: a response carrying neither freshness information nor a
validator is not considered cacheable.