rm tests: actually test for SIGPIPE in SIGPIPE test

rm tests: actually test for SIGPIPE in SIGPIPE test

Change a test initially added in 50cd31c652 (t3600: comment on
inducing SIGPIPE in `git rm`, 2019-11-27) to explicitly test for
SIGPIPE using a pattern initially established in 7559a1be8a (unblock
and unignore SIGPIPE, 2014-09-18).

The problem with using that pattern is that it requires us to skip the
test on MINGW[1]. If we kept the test with its initial semantics[2]
we'd get coverage there, at the cost of not checking whether we
actually had SIGPIPE outside of MinGW.

Arguably we should just remove this test. Between the test added in
7559a1be8a and the change made in 12e0437f23 (common-main: call
restore_sigpipe_to_default(), 2016-07-01) it's a bit arbitrary to only
check this for "git rm".

But in lieu of having wider test coverage for other "git" subcommands
let's refactor this to explicitly test for SIGPIPE outside of MinGW,
and then just that we remove the ".git/index.lock" (as before) on all
platforms.

1. https://lore.kernel.org/git/xmqq1rec5ckf.fsf@gitster.c.googlers.com/
2. 0693f9ddad (Make sure lockfiles are unlocked when dying on SIGPIPE,
   2008-12-18)

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