test-lib: add a GIT_TEST_PASSING_SANITIZE_LEAK=check mode

test-lib: add a GIT_TEST_PASSING_SANITIZE_LEAK=check mode

Add a new "GIT_TEST_PASSING_SANITIZE_LEAK=check" mode to the
test-lib.sh.

As noted in the updated "t/README" this compliments the existing
"GIT_TEST_PASSING_SANITIZE_LEAK=true" mode added in
956d2e4639b (tests: add a test mode for SANITIZE=leak, run it in CI,
2021-09-23).

Rather than document this all in one (even more) dense paragraph split
up the discussion of how it combines with --immediate into its own
paragraph following the discussion of
"GIT_TEST_SANITIZE_LEAK_LOG=true".

Before the removal of "test_external" in a preceding commit we would
have had to special-case t9700-perl-git.sh and t0202-gettext-perl.sh.

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