Add test for string/butlast

This adds a test for the `string/butlast` instruction

test/propeller/push/instructions/string_spec.clj

@@ -0,0 +1,27 @@
+(ns propeller.push.instructions.string-spec
+  (:require
+   [clojure.test.check.generators :as gen]
+   [clojure.test.check.properties :as prop]
+   [clojure.test.check.clojure-test :as ct :refer [defspec]]
+   [propeller.push.state :as state]
+   [propeller.push.core :as core]
+   [propeller.push.instructions.string :as string]))
+
+
+;; string/butlast
+
+(defn check-butlast
+  [value]
+  (let [start-state (state/push-to-stack state/empty-state
+                                         :string
+                                         value)
+        end-state ((:string_butlast @core/instruction-table) start-state)
+        expected-result (apply str (butlast value))]
+    (= expected-result
+       (state/peek-stack end-state :string))))
+
+(defspec butlast-spec 100
+  (prop/for-all [s gen/string] 
+                (check-butlast s)))
+
+