Test for receiver is too slow

[catkira]

The test for receiver.sv currently simulations 45 ms of input samples at a sample rate of 3.84 Hz (256-FFT). Since this test has to run for different parameter configurations it makes the CI run very long (>2h). I expect a big speedup from Verilator simulations if the test runs in pure C++ without Python/Cocotb. Outputs (IQ data) can be stored in txt files and then used by a separate Python script for visualization.