Location of functional testing assets

[gryphonmyers]

Use of Unreal's functional testing features involves the creation of *.umap files that are intended to be run with the automation tools, not played directly by developers or users. Currently the guide explicitly states that ALL map/level files should go in a single directory, but I'd suggest that functional test levels should be called out as an exception to this rule.

More generally, it might be good to establish convention for storage of tests and fixtures in general for those following TDD.