tests: change "mkdir -p && write_script" to use "test_hook"

tests: change "mkdir -p && write_script" to use "test_hook"

Change tests that used a "mkdir -p .git/hooks && write_script" pattern
to use the new "test_hook" helper instead. The new helper does not
create the .git/hooks directory, rather we assume that the default
template will do so for us.

An upcoming series[1] will extend "test_hook" to operate in a
"--template=" mode, but for now assuming that we have a .git/hooks
already is a safe assumption. If that assumption becomes false in the
future we'll only need to change 'test_hook", instead of all of these
callsites.

1. https://lore.kernel.org/git/cover-00.13-00000000000-20211212T201308Z-avarab@gmail.com/

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