canonicalize test failures on Cygwin

[Bruno Haible]

Hi Paul,

Some of the new tests that you added to test-canonicalize.c and
test-canonicalize-lgpl.c on 2020-12-24 fail on Cygwin 2.9:

  /* Check that a non-directory symlink with trailing slash yields NULL,
     and likewise for other troublesome suffixes.  */
  {
    char const *const file_name[]
      = {
         BASE "/huk/",
         BASE "/huk/.",
         BASE "/huk/./",
         BASE "/huk/./.",
         BASE "/huk/x",
         BASE "/huk/..",
         BASE "/huk/../",
         BASE "/huk/../.",
         BASE "/huk/../x",
         BASE "/huk/./..",
         BASE "/huk/././../x",
        };
    for (int i = 0; i < sizeof file_name / sizeof *file_name; i++)
      {
        ...

The tests in the 'for' loop fail for i = 5 ... 10. That is, something
with the ".." handling is not working as expected.

Do you have access to a Cygwin machine for investigation?

Bruno