Allow using input for assert arguments

[R9295]

---
type: "SERIALIZE"
tests:
  test_something:
      info: "Example Test"
      input:
        type: "str"
        arg: "hello, world"
      assert:
        func: "assert_list_entries"
       # Allow this
        arg:
            type: "str"
            arg: "hello, world"

It should be possible to use input in assert's argument. As auto-formatting string or loading file content would help a lot