Added more rbyd fuzz testing

- Added test_rbyd_fuzz_mixed/test_rbyd_fuzz_sparse
- Added test_rbyd_unwritten_mixed_fuzz/test_rbyd_unwritten_sparse_fuzz
- Also renamed "random" tests to "fuzz", this describes their purpose a
  bit better

These were a bit tricky to add since they need to simulate rbyd weights,
but they should give significant coverage over complicated rbyd corner
cases I may have not thought about.

Also fixed a miscalculation in lfsr_rbyd_pendinglookup when finding a
id that grew. Finding this bug is a good sign these tests are working.

lfs.c

@@ -1663,9 +1663,9 @@ again:;
             // follow the upper edge of the grow
             } else if (attr->id <= id__
                     && attr->id+(lfs_ssize_t)attr->size > id__) {
+                id += attr->id+attr->size - id__;
                 id__ = attr->id;
                 tag = 0x10;
-                id += id__-attr->id+1;
 
             // adjust ids
             } else if (attr->id <= id__) {