fix yaml map's equality

[suzuki-shunsuke]

---
entries:
- name: main
  script_file: foo.tengo
  params: {}
  tests:
  - name: test
    var_name: result
    equal:
      foo: hello

foo.tengo

result := {
  foo: "hello"
}

$ tengo-tester run
test fails
entry_name: main
test_name: test
the value of the variable result is unexpected (-want, +got)
interface{}(
-       map[interface{}]interface{}{string("foo"): string("hello")},
+       map[string]interface{}{"foo": string("hello")},
  )
entry fails
entry_name: main

FATA[0000]

[suzuki-shunsuke]

https://github.com/d5/tengo/blob/master/docs/tutorial.md#values-and-value-types

The type of key of Tengo's map should be string.

go-yaml/yaml#139

[suzuki-shunsuke]

https://github.com/suzuki-shunsuke/go-dataeq

json: unsupported type: map[interface {}]interface {}

[suzuki-shunsuke]

https://github.com/suzuki-shunsuke/go-convmap