docs: Add new Testing guide

It's pretty bare-bones at the moment, but does close #476 by mentioning
that `bcrypt_elixir` can be tweaked for faster tests

documentation/dsls/DSL:-AshAuthentication.Strategy.Password.cheatmd

@@ -91,6 +91,10 @@ Dispatching to plugs directly:
     ...> conn = Strategy.plug(strategy, :reset_request, conn)
     ...> {_conn, :ok} = Plug.Helpers.get_authentication_result(conn)
 
+## Testing
+
+See the [Testing guide](/documentation/topics/testing.md) for tips on testing resources using this strategy.
+
 ## DSL Documentation
 
 Strategy for authenticating using local resources as the source of truth.

documentation/topics/testing.md

@@ -0,0 +1,14 @@
+# Testing
+
+Tips and tricks to help test your apps.
+
+## When using the Password strategy
+
+AshAuthentication uses `bcrypt_elixir` for hashing passwords for secure storage, which by design has a high computational cost. To reduce the cost (make hashing faster), you can reduce the number of computation rounds it performs in tests:
+
+```elixir
+# in config/test.exs
+
+# Do NOT set this value for production
+config :bcrypt_elixir, log_rounds: 1
+```

lib/ash_authentication/strategies/password.ex

@@ -90,6 +90,10 @@ defmodule AshAuthentication.Strategy.Password do
       ...> conn = Strategy.plug(strategy, :reset_request, conn)
       ...> {_conn, :ok} = Plug.Helpers.get_authentication_result(conn)
 
+  ## Testing
+
+  See the [Testing guide](/documentation/topics/testing.md) for tips on testing resources using this strategy.
+
   ## DSL Documentation
 
   #{Spark.Dsl.Extension.doc_entity(Dsl.dsl())}