tests: fix a memory leak in test-parse-options.c

Fix a memory leak in t/helper/test-parse-options.c, we were not
freeing the allocated "struct string_list" or its items. Let's move
the declaration of the "list" variable into the cmd__parse_options()
and release it at the end.

In c8ba163916 (parse-options: add OPT_STRING_LIST helper, 2011-06-09)
the "list" variable was added, and later on in
c8ba163916 (parse-options: add OPT_STRING_LIST helper, 2011-06-09)
the "expect" was added.

The "list" variable was last touched in 2721ce21e4 (use string_list
initializer consistently, 2016-06-13), but it was still left at the
static scope, it's better to move it to the function for consistency.

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

t/t0040-parse-options.sh

@@ -5,6 +5,7 @@
 
 test_description='our own option parser'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 cat >expect <<\EOF