Add exit_code and status to Testruns, create model for TestrunMessages

* This is the first step to migrate the `output` column from Testruns to a dedicated table TestrunMessages

app/models/testrun.rb

@@ -4,4 +4,16 @@ class Testrun < ApplicationRecord
   belongs_to :file, class_name: 'CodeOcean::File', optional: true
   belongs_to :submission
   belongs_to :testrun_execution_environment, optional: true, dependent: :destroy
+  has_many :testrun_messages, dependent: :destroy
+
+  enum status: {
+    ok: 0,
+    failed: 1,
+    container_depleted: 2,
+    timeout: 3,
+    out_of_memory: 4,
+  }, _default: :ok, _prefix: true
+
+  validates :exit_code, numericality: {only_integer: true, min: 0, max: 255}, allow_nil: true
+  validates :status, presence: true
 end