Add ability to generate random data for tests

[dpfg]

Could be useful to have an ability to generate some arguments on the fly. E.g. timestamp.

[dpfg]

Double curly braces could be an extension to an {argName}

"args": {
  "base": "{currency}", // arg
  "iso_now": "{{ isoTimestamp }}", // function
  "unix_now": "{{ timestamp }}" // function
}

[vadimkonovod]

Faced with the need for such improvement when were testing resource "changed" endpoint.
It would be cool to have an ability to set a timestamp for "since" query param dynamically :
../changed?since={{ isoNow() }}

Also it would be great to add some math operations on timestamp values. It will allow to test TimeZone processing:
../changed?since={{ isoNow("UTC", -3) }} or like this
../changed?since={{ isoNow(hh, -3) }}