MDL-47497 behat: steps for basic download testing.

These steps were created by sam marshall and Jes Ackland-Snow at the Open
University. I just altered them to make them a bit more generic, and
submitted them to Moodle core, at the point when I wanted to reuse them
in another plugin.

This commit introduces two new steps:

Then following "XXX" should download "YY" bytes
Then following "XXX" should download between "YY" and "ZZ" bytes

At the moment, this only works for links with href attributes.
MDL-47497 qformats gift and xml: basic test of export.

Really, this is just to test the new behat setps, as much as anything.

Note that the details of what is written to the export files is tested in
the unit tests.