t0040-parse-options: test parse_options() with various 'parse_opt_flags'

In 't0040-parse-options.sh' we thoroughly test the parsing of all
types and forms of options, but in all those tests parse_options() is
always invoked with a 0 flags parameter.

Add a few tests to demonstrate how various 'enum parse_opt_flags'
values are supposed to influence option parsing.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-tool.c

@@ -51,6 +51,7 @@ static struct test_cmd cmds[] = {
 	{ "online-cpus", cmd__online_cpus },
 	{ "pack-mtimes", cmd__pack_mtimes },
 	{ "parse-options", cmd__parse_options },
+	{ "parse-options-flags", cmd__parse_options_flags },
 	{ "parse-pathspec-file", cmd__parse_pathspec_file },
 	{ "partial-clone", cmd__partial_clone },
 	{ "path-utils", cmd__path_utils },