This adds a query_parameters field which can be used to add query
parameters to the url of the test in which it is used. It is a dict in
a form suitable for being passed to the urlencode() method.
This change is cumbersome because urlencoded in python2 and 3 have
different behaviors: In python2 a utf-8 encoded string is required.
In 3 it will figure out the right thing. So we here we just encode.
We also need to deal with the fact that a numeral in YAML will be
provided to Python as a numeric value and we need to stringify that in
a version independent fashion.
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