Tests that allow easy exploration of user objects have been added.

Fixes #3394

These tests are entirely trivial from a test-suite point of view.
They pass if they run.  They fail if the input files cause a crash.

Also a little doco has been added to user.tex that explains the
functionality.
Merge pull request #3402 from WilkAndy/uo_eg_3394

Tests that allow easy exploration of user objects have been added.