tests: run internal chain-linter under "make test"

tests: run internal chain-linter under "make test"

Since 69b9924b875 (t/Makefile: teach `make test` and `make prove` to run
chainlint.pl, 2022-09-01), we run a single chainlint.pl process for all
scripts, and then instruct each individual script to run with the
equivalent of --no-chain-lint, which tells them not to redundantly run
the chainlint script themselves.

However, this also disables the internal linter run within the shell by
eval-ing "(exit 117) && $1" and confirming we get code 117. In theory
the external linter produces a superset of complaints, and we don't need
the internal one anymore. However, we know there is at least one case
where they differ. A test like:

	test_expect_success 'should fail linter' '
		false &&
		sleep 2 &
		pid=$! &&
		kill $pid
	'

is buggy (it ignores the failure from "false", because it is
backgrounded along with the sleep). The internal linter catches this,
but the external one doesn't (and teaching it to do so is complicated[1]).
So not only does "make test" miss this problem, but it's doubly
confusing because running the script standalone does complain.

Let's teach the suppression in the Makefile to only turn off the
external linter (which we know is redundant, as it was already run) and
leave the internal one intact.

I've used a new environment variable to do this here, and intentionally
did not add a "--no-ext-chain-lint" option. This is an internal
optimization used by the Makefile, and not something that ordinary users
would need to tweak.

[1] For discussion of chainlint.pl and this case, see:
    https://lore.kernel.org/git/CAPig+cQtLFX4PgXyyK_AAkCvg4Aw2RAC5MmLbib-aHHgTBcDuw@mail.gmail.com/

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>