Add Testbench structures

Created by: ganoam

• src/acc_test.sv: Holds all shared testbench infrastructures.

• src/acc_adapter_tb.sv: Randomly generated X requests are translated translated to the C interface using randomly generated predecoder responses.

• src/acc_interconnect_tb.sv: One level of the accelerator interconnect is simulated. Direct connections and bypass paths are exercised.

Signed-off-by: ganoam gnoam@live.com