Restore non-redundant `func-test` completions tests

[fendor]

The PR #2838 shows that some of the completions tests from func-test were not redundant but important to ensure correctness. See #2838 (comment) for description.

We should restore these tests and review the testcases in more detail, moving all non-redundant testcases to the ghcide test suite.

The PR that removed them is #3828

[fendor]

As the test wasn't deterministic, I don't want to restore it. The behaviour seems well-defined to me.