feat: usability in unit tests

[fsamin]

Would like to use venom steps in a golang unit tests as following

func TestSomething(t *testing.T) {
    steps := []venom.Step{
        {
             Type: "http",
             Method: "POST",
             URL: "https://eu.api.ovh.com/1.0/auth/logout",
             Assertions: []venom.Assertion{
                 {"result.statuscode", venom.Asserts.ShouldEqual, 401},
             },
        },
    }
    venom.Run(t, steps)
}

or

func TestSomething(t *testing.T) {
    steps := []venom.Step{
        {
             Type: "http",
             Method: "POST",
             URL: "https://eu.api.ovh.com/1.0/auth/logout", //No default assertions at all
        },
    }
    v := venom.Run(t, steps)
    assert.NoError(t, v.Err())
    assert.NoEqual(t, v.Result["statuscode"], 401)
}