libtests: Include the Unicode test files directly.

Rather than fetch the files using `wget' at test time, fire up `gzip' to
decompress them from local copies.  The files compress really rather
well, so this is an overall saving in disk space relative to the
previous version -- especially since we now share the test files among
all build trees rather than having a separate copy in each.  On the
other hand, they're moderately large things to have in the source
distribution, though small compared to the `images/' tree.

Of course, the main reason for doing this is to completely eliminate the
need for external network connectivity during a build.

The copyright notice, at https://www.unicode.org/copyright.html, appears
to be compatible with the GPL (which is good, because I think we'd have
had a problem using these files even if we didn't distribute them).
I've included the copyright notice as COPYING.unicode-tests, in order to
comply with requirement (a).

Should it be necessary to update the copies of the test files, there's a
(slightly hairy) make target `update-unicode-tests' which can be invoked
by hand to do this.