R1:70b008d18da5

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…