test-lib-functions: move test_set_index_version() to its user

Move the test_set_index_version() function to its only user. This
function has only been used in one place since its addition in
5d9fc888b4 (test-lib: allow setting the index format version,
2014-02-23). Let's have that test script define it.

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

t/t2104-update-index-skip-worktree.sh

@@ -9,6 +9,11 @@ test_description='skip-worktree bit test'
 
 sane_unset GIT_TEST_SPLIT_INDEX
 
+test_set_index_version () {
+    GIT_INDEX_VERSION="$1"
+    export GIT_INDEX_VERSION
+}
+
 test_set_index_version 3
 
 cat >expect.full <<EOF