TST: modify the demo_signal test to loop over all available signals.
TST: modify the demo_signal test to loop over all available signals.

This ensures all signals exist in the reference dictionary.

rgommers/pywt,grlee77/pywt,PyWavelets/pywt,rgommers/pywt,PyWavelets/pywt,rgommers/pywt,grlee77/pywt,rgommers/pywt