Merchant route selection: very basic stuff works

Tests pass.

test/the_great_game/merchants/merchants_test.clj

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+(ns the-great-game.merchants.merchants-test
+  (:require [clojure.test :refer :all]
+            [the-great-game.utils :refer [deep-merge]]
+            [the-great-game.world.world :refer [default-world]]
+            [the-great-game.merchants.merchants :refer :all]))
+
+(deftest expected-price-test
+  (testing "Anticipated prices in markets"
+    (let [world (deep-merge
+                  default-world
+                  {:merchants
+                   {:archie
+                    {:known-prices
+                     {:buckie
+                      {:iron
+                       [{:price 1.7 :date 1}
+                        {:price 2 :date 0}]}}}}})]
+      (let [actual (expected-price (-> world :merchants :archie) :fish :edinburgh)
+            expected 1] ;;
+        (is (= actual expected) "if no information assume 1"))
+      (let [actual (expected-price (-> world :merchants :archie) :iron :buckie)
+            expected 1.7] ;;
+        (is (= actual expected) "if information select the most recent")))))