don't try to test missing library

Author: coke

doc/Language/nativecall.pod6

@@ -668,7 +668,7 @@ to be prepended with "lib" and appended with ".so" (or just appended with
 ".dll" on Windows), and will be searched for in the paths in the
 LD_LIBRARY_PATH (PATH on Windows) environment variable.
 
-=begin code
+=begin code :skip-test<don't have library>
 use NativeCall;
 constant LIBMYSQL = 'mysqlclient';
 constant LIBFOO = '/usr/lib/libfoo.so.1';