t/t5705-clone-2gb: change from skip_all=* to prereq skip

Change this test to skip test with test prerequisites, and to do setup
work in tests. This improves the skipped statistics on platforms where
the test isn't run.

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

t/t5705-clone-2gb.sh

@@ -3,12 +3,14 @@
 test_description='Test cloning a repository larger than 2 gigabyte'
 . ./test-lib.sh
 
-test -z "$GIT_TEST_CLONE_2GB" &&
-skip_all="Skipping expensive 2GB clone test; enable it with GIT_TEST_CLONE_2GB=t" &&
-test_done &&
-exit
+if test -z "$GIT_TEST_CLONE_2GB"
+then
+	say 'Skipping expensive 2GB clone test; enable it with GIT_TEST_CLONE_2GB=t'
+else
+	test_set_prereq CLONE_2GB
+fi
 
-test_expect_success 'setup' '
+test_expect_success CLONE_2GB 'setup' '
 
 	git config pack.compression 0 &&
 	git config pack.depth 0 &&
@@ -36,13 +38,13 @@ test_expect_success 'setup' '
 
 '
 
-test_expect_success 'clone - bare' '
+test_expect_success CLONE_2GB 'clone - bare' '
 
 	git clone --bare --no-hardlinks . clone-bare
 
 '
 
-test_expect_success 'clone - with worktree, file:// protocol' '
+test_expect_success CLONE_2GB 'clone - with worktree, file:// protocol' '
 
 	git clone file://. clone-wt