Bug 924833 - Re-enable test_receive_call.py

This test uses Plivo to place a call. I have updated `testvars_template.json` with the required structure, but in order to run the test you'll need Plivo credentials. You can either ping me for a PM with my credentials or you could set up your own free Plivo trial account and use your own.

I'm not sure that the latter option will work in the UK or Romania though, and you might not even be able to place calls to your own phones on those countries with a trial account. I think you need a full account to be able to call international numbers.

If we want to set up an adhoc run on CI for the test (which is a good idea) then we'll have to update the actual credentials files and I guess we can just use my test account for that for now.

Note that this code uses the Plivo API to wait for the call to be placed, connected, and completed, so we shouldn't run into issues of the test proceeding/finishing before Plivo thinks the call has been finished.