Add test for varadic tuples in signature

[oxinabox]

I thought this might be broken, but it seems it wasn't.
During the investigation I wrote this test.
It seems worth keeping around

[codecov]

Codecov Report

Merging #24 (1b4a541) into master (3d15ed4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files           5        5           
  Lines         215      215           
=======================================
  Hits          212      212           
  Misses          3        3           

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