tests: replaced stretchr/testify by go-openapi/testify #97

Job	Run time
Lint	32s
Unit tests (macos-latest, oldstable)	39s
Unit tests (macos-latest, stable)	39s
Unit tests (ubuntu-latest, stable)	37s
Unit tests (windows-latest, oldstable)	2m 28s
Unit tests (ubuntu-latest, oldstable)	35s
Unit tests (windows-latest, stable)	1m 19s
	6m 49s

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