svn tests: refactor away a "set -e" in test body

svn tests: refactor away a "set -e" in test body

Refactor a test added in 83c9433e67 (git-svn: support for git-svn
propset, 2014-12-07) to avoid using "set -e" in the test body. Let's
move this into a setup test using "test_expect_success" instead.

While I'm at it refactor:

 * Repeated "mkdir" to "mkdir -p"
 * Uses of "touch" to creating the files with ">" instead
 * The "rm -rf" at the end to happen in a "test_when_finished"

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>