node tests: Add test for topic_suggestions.

This is a prep commit to help us soon remove 44 months worth of
bit-rotted code.

frontend_tests/node_tests/search_suggestion_now.js

@@ -709,6 +709,49 @@ test("topic_suggestions", ({override, override_rewire}) => {
     assert.deepEqual(suggestions.strings, expected);
 });
 
+test("topic_suggestions (limits)", () => {
+    let candidate_topics = [];
+
+    function assert_result(guess, expected_topics) {
+        assert.deepEqual(
+            search.get_topic_suggestions_from_candidates({candidate_topics, guess}),
+            expected_topics,
+        );
+    }
+
+    assert_result("", []);
+    assert_result("zzz", []);
+
+    candidate_topics = ["a", "b", "c"];
+    assert_result("", ["a", "b", "c"]);
+    assert_result("b", ["b"]);
+    assert_result("z", []);
+
+    candidate_topics = [
+        "a1",
+        "a2",
+        "b1",
+        "b2",
+        "a3",
+        "a4",
+        "a5",
+        "c1",
+        "a6",
+        "a7",
+        "a8",
+        "c2",
+        "a9",
+        "a10",
+        "a11",
+        "a12",
+    ];
+    // We max out at 10 topics, so as not to overwhelm the user.
+    assert_result("", ["a1", "a2", "b1", "b2", "a3", "a4", "a5", "c1", "a6", "a7"]);
+    assert_result("a", ["a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a10"]);
+    assert_result("b", ["b1", "b2"]);
+    assert_result("z", []);
+});
+
 test("whitespace_glitch", ({override_rewire}) => {
     const query = "stream:office "; // note trailing space