use charnames ();
sub escape_str {
    my $str = shift;
    $str =~ s{([^\0-\177])}{_N_escape($1)}eg;
    return $str;
}

sub _N_escape {
    return '\N{' . charnames::viacode(ord($_[0])) . '}';
}

##</code><code>##

sub _print_diag {
    my $self = shift;

    local($\, $", $,) = (undef, ' ', '');
    my $fh = $self->todo ? $self->todo_output : $self->failure_output;
    print $fh @_;
}    

##</code><code>##

$ perl -MTest::More -wle 'binmode Test::More->builder->failure_output, ":utf8"; diag chr(228)'
# ä