Reorganize shared unit test code

Move mocks and test fixtures to the modules that they are mocking:
 - datatest.*, mock_datamodule.* to analysisdata/tests/
 - mock_helptopic.* to onlinehelp/tests/
This is now easier with object libraries, and clarifies the dependencies
in the test code.  src/testutils/ is now really about utilities, and not
about some specific-purpose test code.

Change-Id: Ib0cbe6382813315df65fa65f7fe0233b9b5e2d4e