add -i tests: mark "TODO" depending on GIT_TEST_ADD_I_USE_BUILTIN

Fix an issue that existed before 0527ccb1b5 (add -i: default to the
built-in implementation, 2021-11-30), but which became the default
with that change, we should not be marking tests that are known to
pass as "TODO" tests.

When GIT_TEST_ADD_I_USE_BUILTIN=1 was made the default we started
passing the tests added in 0f0fba2cc8 (t3701: add a test for advanced
split-hunk editing, 2019-12-06) and 1bf01040f0 (add -p: demonstrate
failure when running 'edit' after a split, 2015-04-16).

Thus we've been emitting this sort of output:

	$ prove ./t3701-add-interactive.sh
	./t3701-add-interactive.sh .. ok
	All tests successful.

	Test Summary Report
	-------------------
	./t3701-add-interactive.sh (Wstat: 0 Tests: 70 Failed: 0)
	  TODO passed:   45, 47
	Files=1, Tests=70,  2 wallclock secs ( 0.03 usr  0.00 sys +  0.86 cusr  0.33 csys =  1.22 CPU)
	Result: PASS

Which isn't just cosmetic, but due to issues with
test_expect_failure (see [1]) we could e.g. be hiding something as bad
as a segfault in the new implementation. It makes sense catch that,
especially before we put out a release with the built-in "add -i", so
let's generalize the check we were already doing in 0527ccb1b5 with a
new "ADD_I_USE_BUILTIN" prerequisite.

1. https://lore.kernel.org/git/patch-1.7-4624abc2591-20220318T002951Z-avarab@gmail.com/

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

t/test-lib.sh

@@ -1759,6 +1759,10 @@ test_lazy_prereq SHA1 '
 	esac
 '
 
+test_lazy_prereq ADD_I_USE_BUILTIN '
+	test_bool_env GIT_TEST_ADD_I_USE_BUILTIN true
+'
+
 # Ensure that no test accidentally triggers a Git command
 # that runs the actual maintenance scheduler, affecting a user's
 # system permanently.