t1517: skip svn tests if svn is not installed

t1517: skip svn tests if svn is not installed

The svn tests currently assume that git-svn's option parsing will always
fail the tests because it exits 0 on --help, not 129.  However, in a
future commit, we'll expect it to exit 0 and the tests will then need to
be updated to succeed in some cases and fail in others.

We therefore need to have t1517 determine whether the Subversion Perl
modules are present, since if they are not, git-svn will die on start
and then it needs to continue to expect failure.  Add a stripped down
version of the tests in t/lib-git-svn.sh as a prerequisite we can use
here for our svn tests.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>