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.

db/migrate/20230710130037_require_contributor_on_submission.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class RequireContributorOnSubmission < ActiveRecord::Migration[7.0]
+  def change
+    change_column_null :submissions, :contributor_id, false
+    change_column_null :submissions, :contributor_type, false
+  end
+end

db/migrate/20230710130038_require_submission_on_testrun.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class RequireSubmissionOnTestrun < ActiveRecord::Migration[7.0]
+  def change
+    change_column_null :testruns, :submission_id, false
+    add_foreign_key :testruns, :submissions
+  end
+end

db/migrate/20230710130039_add_user_to_testrun.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class AddUserToTestrun < ActiveRecord::Migration[7.0]
+  def change
+    add_reference :testruns, :user, polymorphic: true, index: true
+
+    up_only do
+      # Since we do not have programming groups, we can assume that the user who triggered a testrun is the same as the author of the corresponding submission.
+      # For programming groups, this assumption is not valid (the author of a submission would the group, whereas an individual user would trigger the testrun).
+      execute <<~SQL.squish
+        UPDATE testruns
+        SET user_id   = submissions.contributor_id,
+            user_type = submissions.contributor_type
+        FROM submissions
+        WHERE submissions.id = testruns.submission_id;
+      SQL
+    end
+
+    change_column_null :testruns, :user_id, false
+    change_column_null :testruns, :user_type, false
+  end
+end