env-helper: move this built-in to "test-tool env-helper"

Since [1] there has been no reason for keeping "git env--helper" a
built-in. The reason it was a built-in to begin with was to support
the GIT_TEST_GETTEXT_POISON mode removed in that commit. I.e. unlike
the rest of "test-tool" it would potentially be called by the
installed git via "git-sh-i18n.sh".

As none of that applies since [1] we should stop carrying this
technical debt, and move it to t/helper/*. As this mostly move-only
change shows this has the nice bonus that we'll stop wasting time
translating the internal-only strings it emits.

Even though this was a built-in, it was intentionally never
documented, see its introduction in [2]. It never saw use outside of
the test suite, except for the "GIT_TEST_GETTEXT_POISON" use-case
noted above.

1. d162b25f95 (tests: remove support for GIT_TEST_GETTEXT_POISON,
   2021-01-20)
2. b4f207f339 (env--helper: new undocumented builtin wrapping
   git_env_*(), 2019-06-21)

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

t/test-lib-functions.sh

@@ -1422,7 +1422,7 @@ test_bool_env () {
 		BUG "test_bool_env requires two parameters (variable name and default value)"
 	fi
 
-	git env--helper --type=bool --default="$2" --exit-code "$1"
+	test-tool env-helper --type=bool --default="$2" --exit-code "$1"
 	ret=$?
 	case $ret in
 	0|1)	# unset or valid bool value