Added fuzz test attribute

This acts as a marker to indicate a fuzz test. It should reference a
define, usually SEED, that can be randomized to get interesting test
permutations.

This is currently unused, but could lead to some interesting uses such
as time-based fuzz testing. It's also just useful for inspecting the
tests (make test-list).

runners/test_runner.h

@@ -35,6 +35,7 @@ struct lfs_config;
 enum test_flags {
     TEST_INTERNAL  = 0x1,
     TEST_REENTRANT = 0x2,
+    TEST_FUZZ      = 0x4,
 };
 typedef uint8_t test_flags_t;