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>