sub-render variables if they contain templates in SetVariableTask and HttpClientTask

[mgor]

this is needed when the value of a variable contains a template (string) that we want to be rendered., e.g.:

Given value for variable "template" is "file://./template.j2.xml"  # template.j2.xml itself contains templates, variable "template" is now the content of the file, hence unrendered

...

Given value for variable "payload" is "{{ template }}"  # this will, runtime render the template and save it in variable "payload"

don't touch global jinja2.filters.FILTERS when testing filters, and make sure to always remove any test filters during tests.