Remove needless :skip-test

The example is only compiled during the test, not run.

Author: zoffixznet

doc/Type/IO/Handle.pod6

@@ -496,7 +496,7 @@ default, implementation-defined buffer size is used.
 See L<flush> to write out data currently in the buffer. Changing buffer
 size flushes the filehandle.
 
-=for code :skip-test
+=for code
 given 'foo'.IO.open: :w, :1000out-buffer {
     .say: 'Hello world!'; # buffered
     .out-buffer = 42;       # buffer resized; previous print flushed