Test addition of sample to non-existant run

Adds a unit test, which I believe should fail, if you try to add a sample with
an invalid run id.  This unit test will need to be reviewed as I haven't worked
on it in several weeks/months and it might be in an intermediate state.

My expectation is that an appropriate foreign key constraint should prevent
such a sample from being inserted. However, in this test, I was able to insert
a Sample with a reference in a missing Run.  I hadn't worked out whether or
this was because my SQLAlchemy model was incorrect, or whether it was that the
file based database being used doesn't support / enforce foreign key
constraints.