test: add --help for command-line options

Add a help message for the test suite, documenting available options,
defaults, and backward-compatible positional arguments.

src/unit_test.h

@@ -74,6 +74,8 @@ struct tf_args {
     int num_processes;
     /* Specific RNG seed */
     const char* custom_seed;
+    /* Whether to print the help msg */
+    int help;
 };
 
 /* --------------------------------------------------------- */