REGTESTS: http_request_buffer: Add a barrier to not mix up log messages

REGTESTS: http_request_buffer: Add a barrier to not mix up log messages

Depending on the timing, time to time, the log messages can be mixed. A
client can start and be fully handled by HAProxy (including its log message)
before the log message of the previous client was emitted or received.  To
fix the issue, a barrier was added to be sure to eval the "expect" rule on
logs before starting the next client.

This patch should fix the issue #1847. It may be backported to all branches
containing this reg-tests.