Default to EMERGENCY_BANNER_REDIS_URL in Banner's Redis

This is a new environment variable we've currently added to Integration
to allow us to using Whitehall's Redis instance, which is shared with
Static.

For environments that don't have this variable set yet (i.e. Staging and
Production) we fallback on the default `REDIS_URL`.

This is temporary - once we've moved all environments over to the new
URL we can remove this default.

`mock_env` is shamelessly copied from
[here](https://gist.github.com/jazzytomato/79bb6ff516d93486df4e14169f4426af)
and allows us to specify some environment variables that won't persist
between tests.

The memoization of the instance variable in the class method was a
tricky one to overcome, as the setup persisted across tests. I've had to
manually clear it on these new tests to get this working.

lib/emergency_banner/display.rb

@@ -3,6 +3,7 @@ class Display
     class << self
       def client
         @client ||= Redis.new(
+          url: ENV.fetch("EMERGENCY_BANNER_REDIS_URL", ENV["REDIS_URL"]),
           reconnect_attempts: [
             15,
             30,

test/unit/emergency_banner/display_test.rb

@@ -7,6 +7,51 @@
     @banner = EmergencyBanner::Display.new
   end
 
+  context ".client" do
+    context "when the EMERGENCY_BANNER_REDIS_URL environment variable has been set" do
+      should "use that value as the URL for the Redis client" do
+        mock_env("EMERGENCY_BANNER_REDIS_URL" => "redis://emergency-banner") do
+          EmergencyBanner::Display.instance_variable_set(:@client, nil)
+
+          Redis.expects(:new).with(
+            url: "redis://emergency-banner",
+            reconnect_attempts: [
+              15,
+              30,
+              45,
+              60,
+            ],
+          )
+
+          EmergencyBanner::Display.client
+        end
+      end
+    end
+
+    context "when the EMERGENCY_BANNER_REDIS_URL environment variable has not been set" do
+      should "use the default REDIS_URL as the URL for the Redis client" do
+        mock_env({
+          "EMERGENCY_BANNER_REDIS_URL" => nil,
+          "REDIS_URL" => "redis://my-redis-url",
+        }) do
+          EmergencyBanner::Display.instance_variable_set(:@client, nil)
+
+          Redis.expects(:new).with(
+            url: "redis://my-redis-url",
+            reconnect_attempts: [
+              15,
+              30,
+              45,
+              60,
+            ],
+          )
+
+          EmergencyBanner::Display.client
+        end
+      end
+    end
+  end
+
   context "#enabled?" do
     should "return enabled is false when redis connection times out and send an error notification" do
       err = Redis::CannotConnectError.new("Timed out connecting to Redis")
@@ -155,5 +200,15 @@
       assert_nil @banner.link_text
     end
   end
+
+  def mock_env(partial_env_hash)
+    old_env = ENV.to_hash
+    ENV.update partial_env_hash
+    begin
+      yield
+    ensure
+      ENV.replace old_env
+    end
+  end
 end
 # rubocop:enable Rails/RefuteMethods