diff.mjs: Fixes tests and adds default flag

    * Sets default flag to `false` for `showSubtraction` flag.
    * Removes extra span for else case that was causing some tests to
    fail. Moreover, the previous behavior was defined as that.
    * Adds custom test for the showSubtraction option, both using the
    `showAdded` and `showRemoved` flags.

src/core/operations/Diff.mjs

@@ -50,7 +50,7 @@ class Diff extends Operation {
             {
                 "name": "Show subtraction",
                 "type": "boolean",
-                "value": true
+                "value": false
             },
             {
                 "name": "Ignore whitespace",
@@ -123,7 +123,7 @@ class Diff extends Operation {
             } else if (diff[i].removed) {
                 if (showRemoved) output += "<span class='hl3'>" + Utils.escapeHtml(diff[i].value) + "</span>";
             } else if (!showSubtraction) {
-                output += Utils.escapeHtml(diff[i].value) + "</span>";
+                output += Utils.escapeHtml(diff[i].value);
             }
         }
 

tests/operations/tests/StrUtils.mjs

@@ -15,7 +15,29 @@ TestRegister.addTests([
         recipeConfig: [
             {
                 "op": "Diff",
-                "args": ["\\n\\n", "Character", true, true, false]
+                "args": ["\\n\\n", "Character", true, true, false, false]
+            }
+        ],
+    },
+    {
+        name: "Diff added with subtraction, basic usage",
+        input: "testing23\n\ntesting123",
+        expectedOutput: "<span class='hl5'>1</span>",
+        recipeConfig: [
+            {
+                "op": "Diff",
+                "args": ["\\n\\n", "Character", true, true, true, false]
+            }
+        ],
+    },
+    {
+        name: "Diff removed with subtraction, basic usage",
+        input: "testing123\n\ntesting3",
+        expectedOutput: "<span class='hl3'>12</span>",
+        recipeConfig: [
+            {
+                "op": "Diff",
+                "args": ["\\n\\n", "Character", true, true, true, false]
             }
         ],
     },