Mention that shrinking is unlikely to work

tests/test_superblocks.toml

@@ -531,6 +531,7 @@ defines.BLOCK_COUNT = 'ERASE_COUNT'
 defines.BLOCK_COUNT_2 = ['ERASE_COUNT/2', 'ERASE_COUNT/4', '2']
 defines.KNOWN_BLOCK_COUNT = [true, false]
 code = '''
+#ifdef LFS_SHRINKIFCHEAP
     lfs_t lfs;
     lfs_format(&lfs, cfg) => 0;
 
@@ -628,6 +629,7 @@ code = '''
     lfs_file_close(&lfs, &file) => 0;
     assert(memcmp(buffer, "hello!", 6) == 0);
     lfs_unmount(&lfs) => 0;
+#endif
 '''
 
 # test that metadata_max does not cause problems for superblock compaction