Add user to testrun

* We want to identify a user that has triggered a testrun. Previously (in regular operation), only submission author who were regular users were able to start a testrun. Now, we want to prepare a future where submission authors are programming groups. Still, a testrun is triggered by an individual user and not a group.
* Further, this commit fixes some missing foreign key constrains.

app/models/user.rb

@@ -17,6 +17,7 @@ class User < ApplicationRecord
   has_many :participations, through: :submissions, source: :exercise, as: :user
   has_many :user_proxy_exercise_exercises, as: :user
   has_many :user_exercise_interventions, as: :user
+  has_many :testruns, as: :user
   has_many :interventions, through: :user_exercise_interventions
   has_many :remote_evaluation_mappings, as: :user
   has_one :codeharbor_link, dependent: :destroy