lazy set: Increase code coverage.

This was pvreviously covered by tests for the buddy list.

We shouldn't rely on other code to complete code coverage
for this module. In the future it would be good to add
more tests to the lazy set tests to directly cover lazy
set functionality.

web/tests/lazy_set.test.js

@@ -23,6 +23,14 @@ run_test("map", () => {
     );
 });
 
+run_test("size", () => {
+    const ls = new LazySet([1, 2]);
+    assert.deepEqual(ls.size, 2);
+
+    ls._make_set();
+    assert.deepEqual(ls.size, 2);
+});
+
 run_test("conversions", () => {
     blueslip.expect("error", "not a number", 2);
     const ls = new LazySet([1, 2]);