Many tests are hard-coded to assume 32 ports

[daall]

Many of the tests (including the DVS setup in conftest) are hard-coded to assume that there will always be 32 active front panel ports in the system.

However, there was a recent change in sonic-net/sonic-buildimage#4499 that made it possible to create persistent DVS instances with <32 ports. Also, we may want to simulate different SKUs in the future that have much greater port densities, especially with multi-ASIC and chassis support coming down the pipeline.

We should update the tests to accommodate a variable number of ports.

See also: #1373