test: 💍 add test for sub-expression variables (#71644)

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

src/plugins/expressions/common/execution/execution.test.ts

@@ -376,6 +376,38 @@ describe('Execution', () => {
         value: 5,
       });
     });
+
+    test('can use global variables', async () => {
+      const result = await run(
+        'add val={var foo}',
+        {
+          variables: {
+            foo: 3,
+          },
+        },
+        null
+      );
+
+      expect(result).toMatchObject({
+        type: 'num',
+        value: 3,
+      });
+    });
+
+    test('can modify global variables', async () => {
+      const result = await run(
+        'add val={var_set name=foo value=66 | var bar} | var foo',
+        {
+          variables: {
+            foo: 3,
+            bar: 25,
+          },
+        },
+        null
+      );
+
+      expect(result).toBe(66);
+    });
   });
 
   describe('when arguments are missing', () => {