vTest: add additional helpful methods to response.go

[MacklinEngineering]

A place for us to add thoughts on which methods we might want to implement into response.go!

[danielledeleo]

Generic methods that take user provided funcs and validates with them. The signature might look something like:

func (r *Response) AssertBodyStringFunc(func(body, val string) bool, val string) *Response
func (r *Response) AssertBodyStringsFunc(func(body, val string) bool, vals []string) *Response