fix custom format checker not found test

[p1c2u]

openapi-schema-validator 0.4.2 fixed the following issue:

• Missing format checker for format no longer raise FormatError

This means it will no longer raise spec OpenAPIValidationError error

Fixes #189

[codecov]

Codecov Report

Merging #190 (9271220) into master (5f45fc6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #190   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files          13       13           
  Lines         420      420           
  Branches       61       61           
=======================================
  Hits          388      388           
  Misses         22       22           
  Partials       10       10           

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