t/lib-git-svn.sh: improve svnserve tests with parallel make test

t/lib-git-svn.sh: improve svnserve tests with parallel make test

Setting SVNSERVE_PORT enables several tests which require a local
svnserve daemon to be run (in t9113 & t9126).  The tests share setup of
the local svnserve via `start_svnserve()`.  The function uses svnserve's
`--listen-once` option, which causes svnserve to accept one connection
on the port, serve it, and exit.  When running the tests in parallel
this fails if one test tries to start svnserve while the other is still
running.

Use the test number as the svnserve port (similar to httpd tests) to
avoid port conflicts.  Developers can set GIT_TEST_SVNSERVE to any value
other than 'false' or 'auto' to enable these tests.

Acked-by: Eric Wong <e@80x24.org>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Todd Zullinger <tmz@pobox.com>