[9.x] Testing methods. Making error messages with json_encode more readable

[ar2r]

Added JSON_UNESCAPED_UNICODE in json_encode to make error messages more readable.

Before patching, error messages looks like this (look at Unicode \u0000):

Failed asserting that exception message 'Unable to find JSON fragment: 

[{"id":1}]

within

[[{"foo":"bar","id":10},{"foo":"\u041c\u0438\u0440","id":30},{"foo":"\u041f\u0440\u0438\u0432\u0435\u0442","id":20}]].

After patching, error messages become more readable:

Failed asserting that exception message 'Unable to find JSON fragment: 

[{"id":1}]

within

[[{"foo":"bar","id":10},{"foo":"Мир","id":30},{"foo":"Привет","id":20}]].

This feature allows reading Unicode text without converting it in external tools or sites.

PS: I removed redundant spaces.