[5.8] Fix assertJsonValidationErrors with muliple messages

[MoonHyuk]

assertJsonValidationErrors only tests first message of given array.

for example,

$this->assertJsonValidationErrors([
    'body' => 'required',
    'receiver' => 'required'
]);

This test seems to work well, but the test below passes too.

$this->assertJsonValidationErrors([
    'body' => 'required',
    'receiver' => 'requiredddddd'
]);

And it's wierd because there is no error message like 'requiredddddd'

Changes made