TST:MNT: Ignore ConvergenceWarning when intentionally not converging

[scottclowe]

Fixes 78385ee.

[codecov]

Codecov Report

Merging #211 (71a30db) into master (21119d3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   93.26%   93.26%           
=======================================
  Files           8        8           
  Lines         921      921           
  Branches      187      187           
=======================================
  Hits          859      859           
  Misses         32       32           
  Partials       30       30           

Flag	Coverage Δ
nbsmoke	66.88% <ø> (ø)
unittests	92.83% <ø> (ø)

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