Added some metadata_max testing

- Added METADATA_MAX to test_runner.
- Added METADATA_MAX to bench_runner.
- Added a simple metadata_max test to test_superblocks, for lack of
  better location.

There have been several issues floating around related to metadata_max
and LFS_ERR_NOSPC which makes me think there's a bug in our metadata_max
logic.

metadata_max was a quick patch and is relatively untested, so an
undetected bug isn't too surprising. This commit adds at least some
testing over metadata_max.

Sure enough, the new test_superblocks_metadata_max test reveals a
curious LFS_ERR_NAMETOOLONG error that shouldn't be there.

More investigation needed.

runners/bench_runner.c

@@ -1322,6 +1322,7 @@ void perm_run(
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
         .compact_thresh     = COMPACT_THRESH,
+        .metadata_max       = METADATA_MAX,
         .inline_max         = INLINE_MAX,
     };