Added sparse B-tree name tests, fixed weight-calculation bug during merge

More proof the tests are working.

This bug was in the code that does an extra lookup for the was-split entry
during merge, so we make sure we have the right id to attach the split
name to. Humorously, this code was already set up correctly, the
"split_id" just wasn't actually used. Unfortunately since
lfsr_rbyd_lookup uses out-pointers to return multiple things the
compiler couldn't detect the unused variable.

lfs.c

@@ -3603,7 +3603,7 @@ static int lfsr_btree_commit(lfs_t *lfs,
             }
 
             err = lfsr_rbyd_append(lfs, &rbyd_,
-                    LFSR_TAG_MKBRANCH, (sdelta == 0 ? sweight : rweight),
+                    LFSR_TAG_MKBRANCH, split_id,
                     LFSR_DATA_DISK(parent.block, split_off, split_size));
             if (err) {
                 assert(!err);