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_message.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+class TestrunMessage < ApplicationRecord
+  belongs_to :testrun
+
+  enum cmd: {
+    input: 0,
+    write: 1,
+    clear: 2,
+    turtle: 3,
+    turtlebatch: 4,
+    render: 5,
+    exit: 6,
+    timeout: 7,
+    out_of_memory: 8,
+    status: 9,
+    hint: 10,
+    client_kill: 11,
+    exception: 12,
+    result: 13,
+  }, _default: :write, _prefix: true
+
+  enum stream: {
+    stdin: 0,
+    stdout: 1,
+    stderr: 2,
+  }, _prefix: true
+
+  validates :cmd, presence: true
+  validates :timestamp, presence: true
+
+  validate :either_data_or_log
+
+  def either_data_or_log
+    if [data, log].count(&:present?) > 1
+      errors.add(log, "can't be present if data is also present")
+    end
+  end
+  private :either_data_or_log
+end