Test that the key is the one we expect

In the Redis healthcheck we write and read to a key, so we should check
it's doing that to the key we want it to. This also opens up the
potential for having different keys.

spec/lib/govuk_healthcheck/redis_spec.rb

@@ -12,17 +12,27 @@
   end
 
   context "when the database is connected" do
-    before { allow(redis_client).to receive(:set) }
+    before do
+      allow(redis_client)
+        .to receive(:set).with("healthcheck", anything)
+    end
+
     it_behaves_like "a healthcheck"
+
     it "returns OK" do
-      expect(redis_client).to receive(:set).with(anything, anything)
       expect(subject.status).to eq(GovukHealthcheck::OK)
     end
   end
 
   context "when the database is not connected" do
-    before { allow(redis_client).to receive(:set).with(anything, anything).and_raise("error") }
+    before do
+      allow(redis_client)
+        .to receive(:set).with("healthcheck", anything)
+        .and_raise("error")
+    end
+
     it_behaves_like "a healthcheck"
+
     it "returns CRITICAL" do
       expect(subject.status).to eq(GovukHealthcheck::CRITICAL)
     end