Use full JSON stringify when printing results in DRY mode

[bahmutov]

Otherwise can shorten some values (in this case arrays of two numbers)

saving snapshot "checks behavior of binary function add 1" for 
file src/data-driven-spec.js
{ name: 'add',
  behavior: 
   [ { given: [Object], expect: 3 },
     { given: [Object], expect: 4 },
     { given: [Object], expect: 0 },
     { given: [Object], expect: 21 } ] }
    ✓ checks behavior of binary function add