perf: Make -t following_perf respect the usual db path rules.

src/tests.h

@@ -15,6 +15,8 @@ typedef struct _tf_test_options_t
 	const char* exe_path;
 	/** A comma-separated list of tests to run, or NULL. */
 	const char* tests;
+	/** Path to the actual local database for running performance tests against. */
+	const char* db_path;
 } tf_test_options_t;
 
 /**