'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>