Update to a more performant random string implementation

Since `SecureRandom.alphanumeric` is Ruby 2.5 only, we can't use that
for now. Implement a new version until we can get Tracks updated to that
version.

test/test_helper.rb

@@ -1,6 +1,7 @@
 ENV['RAILS_ENV'] ||= 'test'
 require File.expand_path('../../config/environment', __FILE__)
 require 'rails/test_help'
+require 'securerandom'
 
 # set config for tests. Overwrite those read from config/site.yml. Use inject to avoid warning about changing CONSTANT
 {
@@ -10,6 +11,7 @@ require 'rails/test_help'
   "time_zone" => "Amsterdam"  # force UTC+1 so Travis triggers time zone failures
 }.inject( SITE_CONFIG ) { |h, elem| h[elem[0]] = elem[1]; h }
 
+
 class ActiveSupport::TestCase
   # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
   #
@@ -38,13 +40,8 @@ class ActiveSupport::TestCase
   # for validation purposes
   #
   def generate_random_string(length)
-    string = ""
-    characters = %w(a b c d e f g h i j k l m n o p q r s t u v w z y z 1\ 0)
-    length.times do
-      pick = characters[rand(26)]
-      string << pick
-    end
-    return string
+    o = [('a'..'z'), ('A'..'Z'), (0..9)].flat_map(&:to_a)
+    (0...length).map { o[rand(o.length)] }.join
   end
 
   def assert_equal_dmy(date1, date2)