submodule--helper: move "is-active" to a test-tool

Create a new "test-tool submodule" and move the "is-active" subcommand
over to it. It was added in 5c2bd8b77a (submodule--helper: add
is-active subcommand, 2017-03-16), since
a452128a36 (submodule--helper: introduce add-config subcommand,
2021-08-06) it hasn't been used by git-submodule.sh.

Since we're creating a command dispatch similar to test-tool.c itself
let's split out the "struct test_cmd" into a new test-tool-utils.h,
which both this new code and test-tool.c itself can use.

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

t/helper/test-tool-utils.h

@@ -0,0 +1,9 @@
+#ifndef TEST_TOOL_UTILS_H
+#define TEST_TOOL_UTILS_H
+
+struct test_cmd {
+	const char *name;
+	int (*fn)(int argc, const char **argv);
+};
+
+#endif