The reviewboard.datagrids unit tests were all contained in a single

large tests.py. Some work is being done to audit and optimize queries

for the datagrid, which will significantly grow this test suite.
To prepare for that work, this change splits up the test suite into a

file for each tested datagrid and for each tested column.
No code changes have been made to the test suites, with the exception

that two suites were merged together, as they were testing the same

datagrid.

Unit tests pass.