Home Assistant calls turn_on regardless of current state, and applying a
scene re-asserts every captured state, so asserting the alarm on over a
rinse-only AddWashSet_1 rewrote it to _7 -- silently widening the moments
the user picked, with no state change on this switch to point at it. The
write is now refused when the mask is already non-zero.
Distinct from the off-then-on path documented in _add_wash_bit_switch,
where the appliance has no subset left to keep.
Also satisfies the ty check on the new tests: resolve(),
for_device_by_model() and exists_fn are all optional, asserted the way
the other capability tests do.
Marek Tyburec
6c980f927b