Test the submodules inside tabs

The actual modules that sit inside a tab weren't being tested as their own module type.
Now they are.
Ensure a module is visible to exist - for hidden tabbed modules
Componentise test for graph stacks so tehy can be re-used
Completion rate can theoretically be 100%
Remove single use variable
Remove redundant test for tab visibility

These are now covered by testing each tab for it's own module type because all modules are tested for visibility in their "exists" test.
Componentise table headings for re-use
Add tests for availability module
Merge pull request #41 from alphagov/feature/tab-submodule-tests

Feature/tab submodule tests