tests: add a test mode for SANITIZE=leak, run it in CI

While git can be compiled with SANITIZE=leak, we have not run
regression tests under that mode. Memory leaks have only been fixed as
one-offs without structured regression testing.

This change adds CI testing for it. We'll now build and small set of
whitelisted t00*.sh tests under Linux with a new job called
"linux-leaks".

The CI target uses a new GIT_TEST_PASSING_SANITIZE_LEAK=true test
mode. When running in that mode, we'll assert that we were compiled
with SANITIZE=leak. We'll then skip all tests, except those that we've
opted-in by setting "TEST_PASSES_SANITIZE_LEAK=true".

A test setting "TEST_PASSES_SANITIZE_LEAK=true" setting can in turn
make use of the "SANITIZE_LEAK" prerequisite, should they wish to
selectively skip tests even under
"GIT_TEST_PASSING_SANITIZE_LEAK=true". In the preceding commit we
started doing this in "t0004-unwritable.sh" under SANITIZE=leak, now
it'll combine nicely with "GIT_TEST_PASSING_SANITIZE_LEAK=true".

This is how tests that don't set "TEST_PASSES_SANITIZE_LEAK=true" will
be skipped under GIT_TEST_PASSING_SANITIZE_LEAK=true:

    $ GIT_TEST_PASSING_SANITIZE_LEAK=true ./t0001-init.sh
    1..0 # SKIP skip all tests in t0001 under SANITIZE=leak, TEST_PASSES_SANITIZE_LEAK not set

The intent is to add more TEST_PASSES_SANITIZE_LEAK=true annotations
as follow-up change, but let's start small to begin with.

In ci/run-build-and-tests.sh we make use of the default "*" case to
run "make test" without any GIT_TEST_* modes. SANITIZE=leak is known
to fail in combination with GIT_TEST_SPLIT_INDEX=true in
t0016-oidmap.sh, and we're likely to have other such failures in
various GIT_TEST_* modes. Let's focus on getting the base tests
passing, we can expand coverage to GIT_TEST_* modes later.

It would also be possible to implement a more lightweight version of
this by only relying on setting "LSAN_OPTIONS". See
<YS9OT/pn5rRK9cGB@coredump.intra.peff.net>[1] and
<YS9ZIDpANfsh7N+S@coredump.intra.peff.net>[2] for a discussion of
that. I've opted for this approach of adding a GIT_TEST_* mode instead
because it's consistent with how we handle other special test modes.

Being able to add a "!SANITIZE_LEAK" prerequisite and calling
"test_done" early if it isn't satisfied also means that we can more
incrementally add regression tests without being forced to fix
widespread and hard-to-fix leaks at the same time.

We have tests that do simple checking of some tool we're interested
in, but later on in the script might be stressing trace2, or common
sources of leaks like "git log" in combination with the tool (e.g. the
commit-graph tests). To be clear having a prerequisite could also be
accomplished by using "LSAN_OPTIONS" directly.

On the topic of "LSAN_OPTIONS": It would be nice to have a mode to
aggregate all failures in our various scripts, see [2] for a start at
doing that which sets "log_path" in "LSAN_OPTIONS". I've punted on
that for now, it can be added later.

As of writing this we've got major regressions between master..seen,
i.e. the t000*.sh tests and more fixed since 31f9acf9ce2 (Merge branch
'ah/plugleaks', 2021-08-04) have regressed recently.

See the discussion at <87czsv2idy.fsf@evledraar.gmail.com>[3] about
the lack of this sort of test mode, and 0e5bba53af (add UNLEAK
annotation for reducing leak false positives, 2017-09-08) for the
initial addition of SANITIZE=leak.

See also 09595ab381 (Merge branch 'jk/leak-checkers', 2017-09-19),
7782066f67 (Merge branch 'jk/apache-lsan', 2019-05-19) and the recent
936e58851a (Merge branch 'ah/plugleaks', 2021-05-07) for some of the
past history of "one-off" SANITIZE=leak (and more) fixes.

As noted in [5] we can't support this on OSX yet until Clang 14 is
released, at that point we'll probably want to resurrect that
"osx-leaks" job.

1. https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer
2. https://lore.kernel.org/git/YS9OT%2Fpn5rRK9cGB@coredump.intra.peff.net/
3. https://lore.kernel.org/git/87czsv2idy.fsf@evledraar.gmail.com/
4. https://lore.kernel.org/git/YS9ZIDpANfsh7N+S@coredump.intra.peff.net/
5. https://lore.kernel.org/git/20210916035603.76369-1-carenas@gmail.com/

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

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