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/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