Show full reference file path on test failure.

When a test fails, instead of seeing:
```
AssertionError: CML do not match: ('pp_rules', 'lbtim_2.cml')
```

This PR lets you see:
```
AssertionError: CML do not match: /the/full/path/to/my/iris/lib/iris/tests/results/pp_rules/lbtim_2.cml
```

So you can actually load the file, etc. :expressionless: