Add a test which shows the backspace issue

Author: mgazza

output_test.go

@@ -19,3 +19,13 @@ func TestTerminal_Backspace(t *testing.T) {
 
 	assert.Equal(t, "Hello", term.content.Text())
 }
+
+func TestTerminal_Backspace_Foreign_Characters(t *testing.T) {
+	term := New()
+	term.Resize(fyne.NewSize(50, 50))
+	term.handleOutput([]byte("çîß"))
+	assert.Equal(t, "çîß", term.content.Text())
+
+	term.handleOutput([]byte{asciiBackspace})
+	assert.Equal(t, "çî", term.content.Text())
+}