Mock out signal.signal for tests.

[jeromekelleher]

Closes #1216

Hit this problem in msprime recently as well, this is the simplest fix.

[codecov]

Codecov Report

Merging #1217 (13c56ed) into main (409361c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1217   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files          26       26           
  Lines       21518    21518           
  Branches      906      906           
=======================================
  Hits        20168    20168           
  Misses       1312     1312           
  Partials       38       38           

Flag	Coverage Δ
c-tests	92.50% <ø> (ø)
python-c-tests	94.91% <ø> (ø)
python-tests	98.64% <ø> (ø)

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[benjeffery]

LGTM!