t7006: simplify exit-code checks for sigpipe tests

t7006: simplify exit-code checks for sigpipe tests

Some tests in t7006 check for a SIGPIPE result by recording $? and
comparing it with test_match_signal. Before the previous commit, the
command was on the left-hand side of a pipe, and so we had to do some
subshell trickery to extract it.

But now that this is no longer the case, we can do things much more
simply: just run the command directly, using braces to avoid wrecking
the &&-chain, and then record $?. We could almost use test_expect_code
here, but it doesn't know about test_match_signal.

Likewise, for tests which expect success (i.e., not SIGPIPE), we can
just put them in the &&-chain as usual. That even lets us get rid of the
!MINGW check, since the expectation is the same on both sides.

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