BUG:signal: Fix median bias in csd(..., average="median")

[yotarok]

This commit fixes the bug that wrongly computes _median_bias from the
number of frequency instead of the number of frames.
A test is added to check (at least) the same bias is applied for both
cases where x is y, and x is not y.

Reference issue

Closes #15601

What does this implement/fix?

The original CSD with average='median' returns results with incorrect scale when the return value is complex.
This PR fix this behavior and adds a test that calls csd 2 times with x is y setting and x is not y, but x==y setting.
The former expected to return real-valued array, where the latter expected to return complex-valued array (with zeroed imag-part).
This test fails if the scalefactor is wrongly applied depending on whether result is complex or not.

Additional information

[ilayn]

Excuse my blindness but I can't see the detail that makes the difference with the previous behavior; it seems like before code was

if condition:
    Pxy = (median with Some real imag stuff) 
    Pxy /= bias
else:
    Pxy = (median stuff)
    Pxy /= bias

and now division operation is taken out of the branches and applied at the end. Could you please clarify it for me?

[ilayn]

The failures seem unrelated so in it goes. Thanks @yotarok @tupui