Split test_dirs into test_dtree and test_dseek

- test_dtree - Pure directory creation/deletion/move functionality
  testing. This ends up testing the core of littlefs file entry
  manipulation, since directories is all we need for that.

- test_dseek - Tests more of the corner cases specific to directory
  iteration and seeking. This involves an annoying amount of
  interactions with concurrent updates to the filesystem that are
  complicated to test for.

Also generally renaming the "fstree" concept to "dtree". This only
changes dbglfs.py as far as I'm aware. It's useful to have a name for
this thing and "directory tree" fits a bit better than "filesystem tree"
which could be ambiguous when we also have the "metadata tree" as a
different concept.

scripts/dbglfs.py

@@ -888,14 +888,14 @@ def main(disk, mroots=None, *,
         if grmed_dir_dids != grmed_dstart_dids:
             corrupted = True
 
-        # are we going to end up rendering the fstree?
-        fstree = args.get('fstree') or not (
+        # are we going to end up rendering the dtree?
+        dtree = args.get('dtree') or not (
             args.get('config') or args.get('gstate'))
 
         # do a pass to find the width that fits file names+tree, this
         # may not terminate! It's up to the user to use -Z in that case
         f_width = 0
-        if fstree:
+        if dtree:
             def rec_f_width(did, depth):
                 depth_ = 0
                 width_ = 0
@@ -930,7 +930,7 @@ def main(disk, mroots=None, *,
         w_width = (m.ceil(m.log10(max(1, mweight)+1))
             + 2*m.ceil(m.log10(max(1, rweight)+1))
             + 2)
-        if fstree:
+        if dtree:
             print('%-11s  %-*s %-*s  %s' % (
                 'mdir',
                 w_width, 'ids',
@@ -1054,8 +1054,8 @@ def main(disk, mroots=None, *,
                                     w_width, '',
                                     line))
 
-        # print fstree?
-        if fstree:
+        # print dtree?
+        if dtree:
             # only show mdir on change
             pmid = None
             # recursively print directories
@@ -1110,7 +1110,7 @@ def main(disk, mroots=None, *,
                             if did_ not in grmed_dir_dids:
                                 notes.append('orphaned')
 
-                    # print human readable fstree entry
+                    # print human readable dtree entry
                     print('%s%12s %*s %-*s  %s%s%s' % (
                         '\x1b[90m' if color and (grmed or tag == TAG_DSTART)
                             else '',
@@ -1238,9 +1238,9 @@ if __name__ == "__main__":
         action='store_true',
         help="Show the gdelta that xors into the global-state.")
     parser.add_argument(
-        '-f', '--fstree',
+        '-t', '--dtree', '--tree',
         action='store_true',
-        help="Show the filesystem tree (default).")
+        help="Show the directory tree (default).")
     parser.add_argument(
         '-A', '--attrs',
         action='store_true',