Fix distort_random for distributed meshes

Previously, distort_random produced unconnected meshes for distributed meshes. Now we don't move vertices that belong to two or more different processes. The resulting mesh is connected.
No need to keep this block in the testcase if you don't want to use it. If you do want to use it, you can always write the data into an `ostringstream` and then output the content of this stream to `deallog`.