test: Add option to display all available tests

Useful option to avoid opening the large tests.c file just to find
the test case you want to run.

src/unit_test.h

@@ -77,6 +77,8 @@ struct tf_args {
     const char* custom_seed;
     /* Whether to print the help msg */
     int help;
+    /* Whether to print the tests list msg */
+    int list_tests;
     /* Target tests indexes */
     struct tf_targets targets;
 };