tests: Skip lsns/ioctl_ns test if unshare fails

tests: Skip lsns/ioctl_ns test if unshare fails

Some parts of the Debian build infrastructure uses unshare to run the
package build, and that appears to cause a "nested" unshare in the
lsns/ioctl_ns test to fail. Unfortunately the tests then hang at this
point.

Try running unshare before the actual test, and skip the test if unshare
already fails.

[kzak@redhat.com: - add --fork to the test
                  - don't write to stdout/err]

Signed-off-by: Chris Hofstaedtler <zeha@debian.org>
Signed-off-by: Karel Zak <kzak@redhat.com>