[5.4] Add context to the assert json test response method

[nathan-isaac]

The issue with the current assertJson method is that it only gives the expected value in the test failure output.

Any thoughts on this new output?

New test failure output:

Unable to find JSON subset:

{
     'message' => 'Successfully updated post.'
}

within

{
    "title": [
        "The title field is required."
    ]
}.


Failed asserting that an array has the subset Array &0 (
    'message' => 'Successfully updated post.'
).

Current test failure output:

Failed asserting that an array has the subset Array &0 (
    'message' => 'Successfully updated post.'
).