Towards gh-18867.

Test module checklist:

• test_result_type.py
• test_dltisys.py
• test_waveforms.py
• test_fir_filter_design.py
• test_windows.py ENH: signal.windows: add array API support (take 2) #21783
• test_czt.py
• test_max_len_seq.py
• test_wavelets.py (deprecated?)
• test_spectral.py WIP, ENH: signal: test_spectral array API support #19001
• test_ltisys.py
• test_upfirdn.py
• test_filter_design.py
• test_savitzky_golay.py
• test_array_tools.py
• test_signaltools.py (partially done in ENH: array types, signal: delegate to CuPy and JAX for correlations and convolutions #20772)
• test_bsplines.py
• test_peak_finding.py
• test_cont2discrete.py
• test_short_time_fft.py

cc @tylerjereddy @jakevdp

** EDIT ** (ev-br): the list above is out of date; the current status is #20678 (comment)