Force client_encoding=utf8 in tests

Some tests were doing SET NAMES 'UTF8' for the non-ASCII bits, while
others were doing nothing and hoping, so just be consistent and use
UTF8 throughout.
Remove old, redundant Unicode tests

These tests are remnants from before the more thorough data-driven
test were introduced, and make it hard to skip the right tests based
on server_encoding (see next commit).
Skip tests for characters not supported by the server_encoding