tests: replaced stretchr/testify by go-openapi/testify #99

Job	Run time
Lint	19s
Unit tests (ubuntu-latest, oldstable)	37s
Unit tests (ubuntu-latest, stable)	1m 38s
Unit tests (macos-latest, stable)	1m 0s
Unit tests (macos-latest, oldstable)	38s
Unit tests (windows-latest, stable)	1m 22s
Unit tests (windows-latest, oldstable)	1m 29s
	7m 3s

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