Make the printing of complex Unicode in a QString prettier

This also has the advantage of not requiring the use of the locale
codec. Quite an advantage if you're debugging the locale codec. But it's
mostly so that we don't get question marks that hide the difference we
were trying to locate.

[ChangeLog][QtTest] QtTest now prints an escaped version of QStrings
that failed to compare with QCOMPARE. That is, instead of converting
non-printable characters to question marks, QtTest will print the
Unicode representation of the character in question.

Change-Id: I44c1ef3246b188c913dacd3ca4df02581356ea41
Reviewed-by: Lars Knoll <lars.knoll@digia.com>