The test port on the water service shut-off is shown on the upstream side of the valve in this detail:

http://www.cityofwpb.com/engineering/standards/2007_Details_PDFs/WATER_SERVICE/WS-10.pdf

Why would that be? If the tap is left open/leaks/etc., then turning off the valve at the connection to the city main would be required for any remedy.

Test cocks are in place to attach test equipment for evaluating whether the double check assembly is still functional. According to the Ninth Edition of the Manual of Cross-Connection
Control, when testing the double check valve assembly, the tester wants to determine if each of the two check valves holds the minimum acceptable value of 1.0 psid in the direction of flow. Refer to the link for further testing instructions.

http://www.usc.edu/dept/fccchr/Crosstalks/ctsp98.pdf

One would assume that the bypass is used to pressurize the discharge side in order to test the check valves and the pressure is released at the upstream test port.

The double check assembly is for irrigation service such as lawn sprinkler systems and swimming pools at single family residential units and would not normally be used 24 hours per day and may be shut down at any time for maintenance.

The double check assembly is different than a Reduced Pressure Zone Device (RPZD, RPZ or RPZ valve). The RPZD is a type of backflow prevention device used to protect water supplies from contamination. RPZDs may also be known as reduced pressure principle backflow prevention devices, or reduced pressure zone assemblies.