tests: skip some unicode tests if expected output won't match

tests: skip some unicode tests if expected output won't match

The expected output strings from the vshtabletest.c are created on a
modern Linux host where unicode printing support is very good. On older
Linux platforms, or non-Linux platforms, some unicode characters will
not be considered printable. While the vsh table alignment code will
stil do the right thing with escaping & aligning in this case, the
result will not match the test's expected output.

Since we know the code is working correctly, do a check with iswprint()
to validate the platform's quality and skip the test if it fails. This
fixes the test on FreeBSD platforms.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>