Add unit test for cangjie_get_characters results

Added test_cangjie_get_characters_results_order() to tests/test_cangjie.c
Tests the results order of cangjie_get_characters() with code "h*i".
Improved shortcode unit test code style

Modified cangjie_get_characters_by_shortcode().

Instead of calling next multiple times, used the cur pointer to store
the position of current test target. This make the code easier to
modify and maintain.
Test shortcode's order according to frequency

In cangjie_get_characters_by_shortcode(), the unit test was testing
against by order happens to be. It should, however, test against
the order of frequency.
Order the query results

Added "ORDER BY frequency ASC" to the end of query strings used
in cangjie_new_characters and cangjie_new_characters_shortcode.

Since the linked-list is built with an prepend mechanism, the results
will be of reverse order of the query. Which will effectively gives
a "ORDER BY frequency DESC" result.

This commit fixes the default order of results, but it does not provide
a way for user to freely specify sorting logic.

Partially fulfilled enhancement #64.