t5608: avoid say() and use "skip_all" instead for consistency

Printing a message directly to stdout could affect TAP processing
and is not really needed, as there is a standard way to skip all
tests that could be used instead, while printing an equivalent
message.

While at it; update the message to better reflect that since
a85efb5985 (t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool,
2019-11-22), the enabling variable should be a recognized boolean
(ex: true, false, 1, 0) and get rid of the prerequisite that used
to guard all the tests, since "skip_all" is just much faster and
idempotent.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5608-clone-2gb.sh

@@ -5,12 +5,11 @@ test_description='Test cloning a repository larger than 2 gigabyte'
 
 if ! test_bool_env GIT_TEST_CLONE_2GB false
 then
-	say 'Skipping expensive 2GB clone test; enable it with GIT_TEST_CLONE_2GB=t'
-else
-	test_set_prereq CLONE_2GB
+	skip_all='expensive 2GB clone test; enable with GIT_TEST_CLONE_2GB=true'
+	test_done
 fi
 
-test_expect_success CLONE_2GB 'setup' '
+test_expect_success 'setup' '
 
 	git config pack.compression 0 &&
 	git config pack.depth 0 &&
@@ -38,13 +37,13 @@ test_expect_success CLONE_2GB 'setup' '
 
 '
 
-test_expect_success CLONE_2GB 'clone - bare' '
+test_expect_success 'clone - bare' '
 
 	git clone --bare --no-hardlinks . clone-bare
 
 '
 
-test_expect_success CLONE_2GB 'clone - with worktree, file:// protocol' '
+test_expect_success 'clone - with worktree, file:// protocol' '
 
 	git clone "file://$(pwd)" clone-wt