tests: fix description of 'test_set_prereq'

'test_set_prereq's description claims that prereqs can be specified to
'test_expect_code', but that is not the case (it is not meant to run a
test _case_, but a git command), so remove it.

OTOH that description doesn't mention 'test_external' and
'test_external_without_stderr' that do accept prereqs, so mention
them.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -423,7 +423,7 @@ write_script () {
 # - Explicitly using test_have_prereq.
 #
 # - Implicitly by specifying the prerequisite tag in the calls to
-#   test_expect_{success,failure,code}.
+#   test_expect_{success,failure} and test_external{,_without_stderr}.
 #
 # The single parameter is the prerequisite tag (a simple word, in all
 # capital letters by convention).