Configure CEfE generator randomness with RSpec seed

We still have at least one flakey spec [1] and I noticed it depends on
the `verified_school` factory which uses the
`ForEducationCodeGenerator.generate` method to set the `School#code`. On
the offchance this is causing some kind of non-deterministic problem and
to make that generator more deterministic in specs, I thought it would
be worthwhile using the RSpec seed to seed the random number generator
(RNG) used by `ForEducationCodeGenerator.generate`.

The changes in this commit allow the RNG to be overridden and adds file
in `spec/support` to do this in specs so it uses the RSpec seed.

[1]: https://github.com/RaspberryPiFoundation/editor-api/blob/f397e870f2a33cce1f53b9104c52314f5233572c/spec/concepts/school_teacher/invite_spec.rb#L14-L17

Author: floehopper

lib/for_education_code_generator.rb

@@ -3,8 +3,12 @@
 class ForEducationCodeGenerator
   MAX_CODE = 1_000_000
 
+  cattr_accessor :random
+
+  self.random ||= Random.new
+
   def self.generate
-    number = Random.new.rand(MAX_CODE)
+    number = random.rand(MAX_CODE)
     code = format('%06d', number)
 
     code.match(/(\d\d)(\d\d)(\d\d)/) do |m|

spec/support/for_education_code_generator.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+RSpec.configure do |config|
+  config.before(:suite) do
+    puts "ForEducationCodeGenerator randomized with seed #{config.seed}"
+    ForEducationCodeGenerator.random = Random.new(config.seed)
+  end
+end