test-completion-model: test set_visible_providers()

It shows other bugs in CompletionModel (the commented tests). It's
difficult to understand how the UI is not broken.
test-completion-model: test clear()
test-completion-model: second populate

test_second_populate_same_order() is almost OK, an assert must be
commented.

test_second_populate_different_order() was known to fail, so it is
entirely commented.
test-completion-model: populate with several batches
test-completion-model: get_providers()
test-completion-model: n_proposals()
test-completion-model: some tests with iters