tests: fix and add lint for non-portable seq

tests: fix and add lint for non-portable seq

The seq command is not in POSIX, and doesn't exist on
e.g. OpenBSD. We've had the test_seq wrapper since d17cf5f3a3 ("tests:
Introduce test_seq", 2012-08-04), but use of it keeps coming back,
e.g. in the recently added "fetch negotiator" tests being added here.

So let's also add a check to "make test-lint". The regex is aiming to
capture the likes of $(seq ..) and "seq" as a stand-alone command,
without capturing some existing cases where we e.g. have files called
"seq", as \bseq\b would do.

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