Tests automatic lookup of Washington sales tax districts and rates.
This will set up two tax classes. One of them (class A) has only the sales tax. The other (class B) will have an additional, manually created tax.
This will test the following sequence of actions (running process_district_update() after each one):
1. Enter a customer in Washington for which there is not yet a district tax entry. 2. Add a manual tax in class B. 3. Rename the sales taxes. 4. Delete the sales taxes. 5. Change the sales tax rates (to simulate a change in the actual rate). 6. Set the sales tax rate to zero.
The correct result is always for there to be exactly one tax entry for this district in each class, with the correct rate, except after step 6, when the rate should remain at zero (because setting the rate to zero is a way of manually disabling the tax).