t/unit-tests: convert ctype tests to use clar

t/unit-tests: convert ctype tests to use clar

Convert the ctype tests to use the new clar unit testing framework.
Introduce a new function `cl_failf()` that allows us to print a
formatted error message, which we can use to point out which of the
characters was classified incorrectly. This results in output like this
on failure:

    # start of suite 1: ctype
    not ok 1 - ctype::isspace
        ---
        reason: |
          Test failed.
          0x0d is classified incorrectly: expected 0, got 1
        at:
          file: 't/unit-tests/ctype.c'
          line: 36
          function: 'test_ctype__isspace'
        ---
    ok 2 - ctype::isdigit
    ok 3 - ctype::isalpha
    ok 4 - ctype::isalnum
    ok 5 - ctype::is_glob_special
    ok 6 - ctype::is_regex_special
    ok 7 - ctype::is_pathspec_magic
    ok 8 - ctype::isascii
    ok 9 - ctype::islower
    ok 10 - ctype::isupper
    ok 11 - ctype::iscntrl
    ok 12 - ctype::ispunct
    ok 13 - ctype::isxdigit
    ok 14 - ctype::isprint

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>