Add test coverage that our definition of BMP agrees with MySQL

Summary:
Ref T1191. Test that MySQL's rules match those of `phutil_is_utf8_with_only_bmp_characters()`:

  - Build a string with //every// character that we consider to be a BMP character.
  - Write it into MySQL.
  - Read it back out.
  - Make sure MySQL didn't truncate it.

Test Plan: Ran unit test. This test runs pretty quickly (50ms), the string with every character isn't all that enormous.

Reviewers: btrahan, arice

Reviewed By: arice

CC: chad, arice, aran

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D8314