Remove UTF8 BMP unit test and replace it with a general UTF8 test

Summary:
Ref T1191. After utf8mb4 conversion, these tests no longer pass because MySQL allows emoji and gclefs and such.

We could keep these tests running by keeping a `ut8f_bin` table around somewhere, but we have no other use cases for it and it does not seem worth the added complexity. All these BMP-only codepaths are on the way out.

Update the `%s` / `%B` test to make sure it's rejecting invalid byte sequences, which are still not permitted.

Test Plan: Tests now pass.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10621