Make random_example_spec.rb tests time-safe

I noticed that the test would occasionally fail because the time
would be different between the first run and second run of
`GovukSchemas::RandomExample.new(schema: schema).payload`

The `public_updated_at` might be 16:06:03 for one and 16:06:04 for
the other. This is just a natural consequence of the time it takes
to execute the code, and is not an indicator of the randomness not
being deterministic, which is what the test is testing.

govuk_schemas.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.4"
   spec.add_development_dependency "rubocop-govuk", "~> 3.8"
+  spec.add_development_dependency "timecop", "~> 0.9"
   spec.add_development_dependency "yard", "~> 0.8"
 
   spec.required_ruby_version = ">= 2.6"

spec/lib/random_example_spec.rb

@@ -1,3 +1,4 @@
+require "timecop"
 require "spec_helper"
 
 RSpec.describe GovukSchemas::RandomExample do
@@ -27,12 +28,14 @@
 
     it "returns the same output if a seed is detected" do
       schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
-      srand(777) # these srand calls would be in the upstream application
-      first_payload = GovukSchemas::RandomExample.new(schema: schema).payload
-      srand(777)
-      second_payload = GovukSchemas::RandomExample.new(schema: schema).payload
-
-      expect(first_payload).to eql(second_payload)
+      # freeze time to avoid inconsistent `public_updated_at` values between runs
+      Timecop.freeze do
+        srand(777) # these srand calls would be in the upstream application
+        first_payload = GovukSchemas::RandomExample.new(schema: schema).payload
+        srand(777)
+        second_payload = GovukSchemas::RandomExample.new(schema: schema).payload
+        expect(first_payload).to eql(second_payload)
+      end
     end
 
     it "can customise the payload" do