The datagrid unit tests were pretty old, and a bit scattered, with poor

naming and some tests that didn't actually properly test the

functionality. For instance, the datagrid rendering didn't check the

results, allowing errors to appear valid (since it's guaranteed that

exceptions won't be raised), and some of the sandboxing tests were

testing errors that the sandboxing code introduced.
This cleans up much of the testing, and the sandboxing bugs while there.

The test names have been changed to use the proper naming and casing

schemes, with proper docstrings. Some setup code has been removed or

made more efficient. Rendered HTML is more closely inspected.
The sandboxing tests have been folded up into other test suites, with

the code being more self-contained and ensuring the right things are

tested and right messages are logged.
A couple of values returned when sandboxing a custom column have been

fixed up as well. This prevents columns from rendering <td>None</td>

when failing to call render_data() and prevents order_by() error

messages when attempting to filter out a non-sortable field.
A bad ending </a> tag in the HTML for columns was also fixed.

Unit tests pass.