PERF: Redis snapshotting during tests (#15260)

We can fake redis transactions so that `fab!` works for redis and PG
data, but it's too slow to be used indiscriminately. Instead, you can
opt into it with the `use_redis_snapshotting` helper.

Insofar as snapshotting allows us to `fab!` more things, it provides a
speedup.

spec/helpers/redis_snapshot_helper.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module RedisSnapshotHelper
+  def use_redis_snapshotting
+    before(:all) do
+      RedisSnapshot.begin_faux_transaction
+    end
+
+    after(:all) do
+      RedisSnapshot.end_faux_transaction
+    end
+
+    before(:each) do
+      RedisSnapshot.begin_faux_transaction
+    end
+
+    after(:each) do
+      RedisSnapshot.end_faux_transaction
+    end
+  end
+end