refactor tests a bit by putting LineGap-creation functionality into TestUtils.init, and make sure we also add a Unix-style newline to the end of the string if it doesn't already have one

test/test-utils.sml

@@ -1,7 +1,18 @@
 structure TestUtils =
 struct
-  fun init buffer =
-    AppType.init (buffer, 0, 0, Time.now ())
+  fun withUnixLineEndings str =
+    if String.size str > 0 andalso String.sub (str, String.size str - 1) = #"\n" then
+      str
+    else
+      str ^ "\n"
+
+  fun init bufferString =
+    let
+      val bufferString = withUnixLineEndings bufferString
+      val buffer = LineGap.fromString bufferString
+    in
+      AppType.init (buffer, 0, 0, Time.now ())
+    end
 
   fun update (app, cmd) =
     AppUpdate.update (app, cmd, Time.now ())