git svn: migrate tests to use lib-httpd

git svn: migrate tests to use lib-httpd

This allows us to use common test infrastructure and parallelize
the tests.  For now, GIT_SVN_TEST_HTTPD=true needs to be set to
enable the SVN HTTP tests because we reuse the same test cases
for both file:// and http:// SVN repositories.  SVN_HTTPD_PORT
is no longer honored.

Tested under Apache 2.2 and 2.4 on Debian 7.x (wheezy) and
8.x (jessie), respectively.

Cc: Clemens Buchacher <drizzd@aon.at>
Cc: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>