t: detect errors outside of test cases

t: detect errors outside of test cases

We have recently merged a patch series that had a simple misspelling of
`test_expect_success`. Instead of making our tests fail though, this
typo went completely undetected and all of our tests passed, which is of
course unfortunate. This is a more general issue with our test suite:
all commands that run outside of a specific test case can fail, and if
we don't explicitly check for such failure then this failure will be
silently ignored.

Improve the status quo by enabling the errexit option so that any such
unchecked failures will cause us to abort immediately.

Note that for now, we only enable this option for Bash 5 and newer. This
is because other shells have wildly different behaviour, and older
versions of Bash (especially on macOS) are buggy. The list of enabled
shells may be extended going forward.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>