Disable three UTF-8 tests that expose bugs in wcwidth(3) in the

Disable three UTF-8 tests that expose bugs in wcwidth(3) in the
native C libraries of illumos, Oracle Solaris 11, and SunOS 5.10.
While it is useful to catch wcwidth(3) regressions on OpenBSD, the
purpose of the *portable* mandoc regression suite is not to check
the C library of the host system; that would just hide genuine
mandoc portability issues in the noise.  The remaining UTF-8 tests
are still sufficient to establish that mandoc does the right thing.

Issues reported by Sevan Janiyan <venture37 at geeklan dot co dot uk>
after testing on OmniOS.