add test for checking direct return from linear combination of inputs

[katat]

https://github.com/zksecurity/noname/issues/77

It passes the test. Here is the reasoning:

• the public var is always added as a single value var at the beginning.
• the circuit writer keeps these public vars as they are.
• so the public vars can be recovered to a single cell var from linear combination.
• the public output var and return var are two different vars (although there is an assert_eq constraint among them at the end).
• the return var can be a linear combination of multiple vars, which is not an issue as they don't need to be converted to a single cell var.