Ensure redis healthcheck key is unique

At the moment we use the same key for our healthcheck. This poses a
potential race condition problem as we share the Redis database across
all our applications.

Instead we can generate a random key for each request which should
prevent this problem from happening.

lib/govuk_app_config/govuk_healthcheck/redis.rb

@@ -1,3 +1,5 @@
+require "securerandom"
+
 module GovukHealthcheck
   class Redis
     def name
@@ -7,9 +9,11 @@ def name
     def status
       client = ::Redis.new
 
-      client.set("healthcheck", "val")
-      client.get("healthcheck")
-      client.del("healthcheck")
+      key = "healthcheck-#{SecureRandom.hex}"
+
+      client.set(key, "val")
+      client.get(key)
+      client.del(key)
 
       client.close
 

spec/lib/govuk_healthcheck/redis_spec.rb

@@ -9,12 +9,13 @@
   before do
     stub_const("Redis", redis)
     allow(redis).to receive(:new).and_return(redis_client)
+    allow(SecureRandom).to receive(:hex).and_return("abc")
   end
 
   context "when the database is connected" do
     before do
       allow(redis_client)
-        .to receive(:set).with("healthcheck", anything)
+        .to receive(:set).with("healthcheck-abc", anything)
     end
 
     it_behaves_like "a healthcheck"
@@ -27,7 +28,7 @@
   context "when the database is not connected" do
     before do
       allow(redis_client)
-        .to receive(:set).with("healthcheck", anything)
+        .to receive(:set).with("healthcheck-abc", anything)
         .and_raise("error")
     end