WIP: qmtest Support for remote refs in tests

Working with @rmatev, this is a simple MR to add support for a remote file as the reference for a test in qmtest.

To do this I added some logic to the _expandReferenceFileName method to determine if a file is to be accessed via a remote protocol (for the moment only http(s)).

If the file is remote it's first downloaded to a local temporary location to allow the rest of the test code to run as normal. The few methods which use the _expandReferenceFileName method have then been updated to check for these temporary files and clean up after the test has been completed.

This has been tested by replacing the reference file location in one of the qmtests with the same file hosted by a http server on a remote machine and the test passed as expected when the file was available.