Tests: fixed test preconditions in proxy_cache_lock_ssi.t. Made sure that a proper request order exists between /locked and /ssi. Previously, request /locked could be proxied after subrequest /active, losing in the limit_req race. Also, request /locked was not properly delayed, thus proxy cache lock could not happen later in a subrequest. This somewhat reverts 714668aea2b0 as now inapplicable. While there, fixed a typo.