Expand Unicode-compatibility of QtTest's logging

Recent fixing of a TAP test logging issue (itself discovered in the course of other work) brought to light that our teamcity test logger was caught out by Unicode content (outside the BMP).
While reviewing the fix for that, macadder (module maintainer) requested "more thorough testing of correct encoding in testlib's loggers."

The fix in question does give cause to believe the other loggers are OK (given that only teamcity had a problem), but we should at least verify this by adding some more Unicode-stressing test-cases to qtbase/tests/auto/testlib/stelftests/.