Add assert functions for ASCII character

[kevin2kevin2]

There could be functions for ASCII character (char):

• ck_assert_char_eq
• ck_assert_char_ne
• ck_assert_char_lower (by using islower)
• ck_assert_char_upper (by using isupper)
• ck_assert_char_ddigit (decimal)
• ck_assert_char_xdigit (by using isxdigit)
• ck_assert_char_ascii (by using isascii)
• etc. (see #include <ctype.h>)

It is simple and could be useful.