DOC: signal: update the description of the 'n' argument of the bode and freqresp functions.
TST: signal: fix/enhance a few tests:
  * Fixed a cut-and-paste mistake in a test of freqresp--it duplicated a test
    of bode, instead of testing freqresp.
  * Updated the state space tests of bode and freqresp to use a system with a
    known frequency response.
  * Updated an older test of lsim2 to use the warnings.catch_warnings context
    manager.
  * Removed some unused variables.