tests: replaced stretchr/testify by go-openapi/testify #96

Job	Run time
Lint	34s
Unit tests (macos-latest, stable)	40s
Unit tests (windows-latest, oldstable)	1m 51s
Unit tests (macos-latest, oldstable)	38s
Unit tests (windows-latest, stable)	1m 8s
Unit tests (ubuntu-latest, oldstable)	33s
Unit tests (ubuntu-latest, stable)	36s
	6m 0s

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