u-string-list: add unit tests for string-list methods

u-string-list: add unit tests for string-list methods

Unit tests in u-string-list.c does not cover several methods
in string-list, this gap in coverage makes it difficult to
ensure no regressions are introduced in future changes.

Add unit tests for the following methods to enhance coverage:
string_list_remove_empty_items()
unsorted_string_list_has_string()
unsorted_string_list_delete_item()
string_list_has_string()
string_list_insert()
string_list_sort()
string_list_remove()

Signed-off-by: Amisha Chhajed <amishhhaaaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>