Randomise default File Catalog name

Since the introduction of running tests in parallel in the nightlies, tests are randomly failing because they try to write concurrently a XML FileCatalog with the same (default) name. This MR randomises the default name so that each test writes to a unique file. Works for all tests that use IOHelper to set up the persistency.

Should be ported also to master branch and to all legacy branches being tested in the nightlies.