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

src/ssb.tests.h

@@ -95,4 +95,10 @@ void tf_ssb_test_triggers(const tf_test_options_t* options);
 */
 void tf_ssb_test_cli(const tf_test_options_t* options);
 
+/**
+** Test following performance.
+** @param options The test options.
+*/
+void tf_ssb_test_following_perf(const tf_test_options_t* options);
+
 /** @} */