perf: Add a quick test to measure ssb.following performance.

src/ssb.tests.c

@@ -1607,4 +1607,23 @@ void tf_ssb_test_cli(const tf_test_options_t* options)
 	tf_free(blob);
 	tf_free(id);
 }
+
+void tf_ssb_test_following_perf(const tf_test_options_t* options)
+{
+	uv_loop_t loop = { 0 };
+	uv_loop_init(&loop);
+
+	tf_ssb_t* ssb = tf_ssb_create(&loop, NULL, "db.sqlite", NULL);
+
+	uint64_t start = uv_hrtime();
+	const char** ids = tf_ssb_db_get_all_visible_identities(ssb, 2);
+	tf_free(ids);
+	uint64_t end = uv_hrtime();
+	tf_printf("completed in %.3fs\n", (end - start) / 1e9);
+
+	tf_ssb_destroy(ssb);
+
+	uv_run(&loop, UV_RUN_DEFAULT);
+	uv_loop_close(&loop);
+}
 #endif