add a test for 'PersistentVector.delete' (even though the 'PersistentVector.merge' helper function is unimplemented)

fcore/persistent-vector.sml

@@ -550,7 +550,7 @@ struct
   fun delete (start, finish, tree) =
     let
       val matchAfterFinish = nextMatch (finish, tree, 1)
-      val left = splitRight (start, tree)
+      val left = splitLeft (start, tree)
     in
       if matchAfterFinish = ~1 then
         (* there is no match after 'finish', so just split left. 

test/persistent-vector-tests.sml

@@ -284,5 +284,39 @@ struct
            end)
     ]
 
-  val tests = [appendTests, toListTests, splitLeftTests]
+  val deleteTests = describe "PersistentVector.delete"
+    [test
+       "returns the left side of the vector \
+       \when 'finish' is greater than any element in the vector"
+       (fn _ =>
+          let
+            (* arrange *)
+            val inputList =
+              [ {start = 1, finish = 1}
+              , {start = 2, finish = 2}
+              , {start = 3, finish = 3}
+              , {start = 4, finish = 4}
+              , {start = 5, finish = 5}
+              , {start = 6, finish = 6}
+              , {start = 7, finish = 7}
+              , {start = 8, finish = 8}
+              ]
+            val pv = PersistentVector.fromList inputList
+
+            (* act *)
+            val pv = PersistentVector.delete (5, 9, pv)
+
+            (* assert *)
+            val outputList = PersistentVector.toList pv
+            val expectedOutput =
+              [ {start = 1, finish = 1}
+              , {start = 2, finish = 2}
+              , {start = 3, finish = 3}
+              , {start = 4, finish = 4}
+              ]
+          in
+            Expect.isTrue (outputList = expectedOutput)
+          end)]
+
+  val tests = [appendTests, toListTests, splitLeftTests, deleteTests]
 end