Change the git-svn tests to use $PERL_PATH, not the "perl" in $PATH.

Using perl in $PATH was added by Sam Vilain in v1.6.6-rc0~95^2~3,
Philippe Bruhat introduced $PERL_PATH to the test suite in
v1.6.6-rc0~9^2, but the lib-git-svn.sh tests weren't updated to use
the new convention.

This resulted in the git-svn tests always being skipped on my
system. My /usr/bin/perl has access to SVN::Core and SVN::Repos, but
the perl in my $PATH does not.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>