As a member of proxy-maint, I frequently want to test user-submitted ebuilds but at the same time I don't want to waste time and failures testing their dependencies. Therefore, I would really appreciate an option that would enable FEATURES=test for packages specified directly on the command-line while merging their dependencies without (added) FEATURES=test.
You can get close to this by using emerge --onlydeps --with-test-deps and then doing a separate emerge invocation for the thing that you want to test.