REGTESTS: http-rules: one connection per request in the replace-uri test

REGTESTS: http-rules: one connection per request in the replace-uri test

The verification campaign caught this test with "HTTP header is
incomplete" when the whole suite runs with "--j 32" on a loaded
machine. Its first client was sending its six requests over a single
connection while the server answers each of them in its own iteration,
so the server closed right when the next request was sent over the
connection haproxy reuses, and the client got that close instead of a
response.

The test is about rewriting the request target, not about keep-alive,
so the six requests are now sent by six clients, each on its own
connection, which is also what the three other clients of the test
already do. Twenty-four runs in parallel with the machine loaded pass.