Add CheckBox status assertions

// Is the expected checkbox checked?
assertChecked("Checked checkbox");
assertChecked(R.string.checked_checkbox);
assertChecked(R.id.checked_checkbox);

// ...And the other checkbox unchecked?
assertUnchecked("Unchecked checkbox");
assertUnchecked(R.string.unchecked_checkbox);
assertUnchecked(R.id.unchecked_checkbox);