Okay, time for some more eclipse tricks. Create new project, do not click finish yet! Uncheck the default location and browser and select the parent project folder and in the address bar of project location type /test and you will see:
Correction: Name Project ParentProjectNameTest, ADT plugin will show an extra project at workspace root(you can ignore that as its not an actual project ..ie its not in file explorer on your SystemOS etc.)
If you use build scripts than the tested.project.dir property will change with the test build script in the test directory as it will be a relative location to the parent project. If you use a different IDE, I am assuming you can do the same trick with the IDE Android plugin you have.
Well, if you are using a DVCS like Git you are already using the do not default location to get all your DVCS stuff in one sub-folder for all projects anyway and thus its just an extension of that technique.