add basic http clone/fetch tests

add basic http clone/fetch tests

This was mostly being tested implicitly by the "http push"
tests. But making a separate test script means that:

  - we will run fetch tests even when http pushing support
    is not built

  - when there are failures on fetching, they are easier to
    see and isolate, as they are not in the middle of push
    tests

This script defaults to running the webserver on port 5550,
and puts the original t5540 on port 5540, so that the two
can be run simultaneously without conflict (but both still
respect an externally set LIB_HTTPD_PORT).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>