Fixed incorrect cache reuse when seeking from end-of-block

In v2.5, we introduced an optimization to avoid rereading data when
seeking inside the file cache. Unfortunately this used a slightly
wrong condition to check if the cache was "live", which meant seeks from
end-of-blocks could end up with invalid caches and wrong data. Not
great.

The problem is the nuance of when a file's cache is "live":

1. The file is marked as LFS_F_READING or LFS_F_WRITING.

   But we can't reuse the cache when writing, so we only care about
   LFS_F_READING.

2. file->off != lfs->cfg->block_size (end-of-block).

   This is an optimization to avoid eagerly reading blocks we may not
   actually care about.

We weren't checking for the end-of-block case, which meant if you seeked
_from_ the end of a block to a seemingly valid location in the file
cache, you could end up with an invalid cache.

Note that end-of-block may not be powers-of-two due to CTZ skip-list
pointers.

---

The fix is to check for the end-of-block case in lfs_file_seek. Note
this now matches the need-new-block logic in lfs_file_flushedread.

This logic change may also make lfs_file_seek call lfs_file_flush more
often, but only in cases where lfs_file_flush is a noop.

I've also extended the test_seek tests to cover a few more boundary-read
cases and prevent a regression in the future.

Found by wjl and lrodorigo

lfs.c

@@ -3725,13 +3725,8 @@ static lfs_soff_t lfs_file_seek_(lfs_t *lfs, lfs_file_t *file,
 
     // if we're only reading and our new offset is still in the file's cache
     // we can avoid flushing and needing to reread the data
-    if (
-#ifndef LFS_READONLY
-        !(file->flags & LFS_F_WRITING)
-#else
-        true
-#endif
-            ) {
+    if ((file->flags & LFS_F_READING)
+            && file->off != lfs->cfg->block_size) {
         int oindex = lfs_ctz_index(lfs, &(lfs_off_t){file->pos});
         lfs_off_t noff = npos;
         int nindex = lfs_ctz_index(lfs, &noff);