Allow optional X selection copy/paste on Linux

As an X11 greybeard I use select-to-copy/middle-click-to-paste more than the standard clipboard. This patch lets us specify 'xsel=True' as an optional argument to copy() and paste() to enable that behaviour.

- Add optional argument to also copy into the X primary selection
  for middle-click copy/paste ability on Linux.
- Use unique text for each test run