s6-test evaluates an expression and indicates the result via its exit status.
s6-test acts as the generic test utility, but it diverges from the specification on one point: if an argument starts with a backslash, this backslash is ignored (and the argument cannot be an operation). This is a simple disambiguation technique that has unfortunately not been chosen by the standard.
s6-test accepts an arbitrary number of arguments.
s6-test is not suitable as a Single Unix test program; however, if your arguments never start with a backslash, it exhibits the exact same behaviour.