t/helper/test-hashmap.c: avoid using `strtok()`

t/helper/test-hashmap.c: avoid using `strtok()`

Avoid using the non-reentrant `strtok()` to separate the parts of each
incoming command. Instead of replacing it with `strtok_r()`, let's
instead use the more friendly pair of `string_list_split_in_place()` and
`string_list_remove_empty_items()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>