In addition to not accepting "-1" as a valid unsigned value, we also
must not format unsigned values as signed ones, i.e. we need to use
wxNumberFormatter::ToString() overload taking unsigned for them.
Do it in wxIntegerValidatorBase::ToString(), used by TransferToWindow(),
and add more tests for the latter.
Also reorganize the tests in sections and use REQUIRE() for the checks
that should prevent the rest of the section from running if they fail.
bdaca38904