Add some basic testing of issue_comment model

A factory and a basic test of validation for the issue_comment model, similar to what we have for diary_comment.

test/factories/issue_comment.rb

@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :issue_comment do
+    sequence(:body) { |n| "This is issue comment #{n}" }
+
+    issue
+    user
+  end
+end

test/models/issue_comment_test.rb

@@ -1,7 +1,9 @@
 require "test_helper"
 
 class IssueCommentTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  test "body must be present" do
+    comment = build(:issue_comment, :body => "")
+    assert_not comment.valid?
+    assert_not_nil comment.errors[:body]
+  end
 end