singleusergroup_tests: Move plugin dependency from deps to runtime_deps
singleusergroup_tests: Move plugin dependency from deps to runtime_deps

The singleusergroup__plugin dependency is incorrectly declared as
compiled dependency, even though it is only runtime dependency. This
was flagged by unused_deps utility.

Change-Id: I89b8c94bc44673d7bdfd69c6ca35383f01d48ee6

GerritCodeReview/plugins_singleusergroup