openapi3(tests): use testify's YAMLEq function

[percivalalb]

Use https://pkg.go.dev/github.com/stretchr/[email protected]/require#YAMLEq which is already in use by other test files.

[alb@192 kin-openapi]$ git grep -i "yamlEq"
openapi2conv/issue187_test.go:  require.YAMLEq(t, string(spec3), expected)
openapi3/issue883_test.go:              require.YAMLEq(t, `
openapi3/issue883_test.go:              require.YAMLEq(t, spec, string(marshalledYaml))
openapi3/issue883_test.go:              require.YAMLEq(t, `
openapi3/issue883_test.go:              require.YAMLEq(t, `
openapi3/issue883_test.go:              require.YAMLEq(t, spec, string(marshalledYaml))

[fenollp]

Thanks!