Pass dimension_separator on fixture generation (fix zarr-developers#858)

Under some test conditions (conda-forge, Debian builds), the
fixtures directory is not available and is then re-created.
When dimension_separator is not passed to DirectoryStore, then
no metadata is assigned to the file. For the "flat" (as opposed
to "flat_legacy") fixture, this meant that the NestedDirectoryStore
did not correct its behavior leading to a failure.

see:
 - conda-forge/zarr-feedstock#56
 - zarr-developers#858

Author: joshmoore

zarr/tests/test_dim_separator.py

@@ -2,6 +2,7 @@
 
 import pytest
 from numpy.testing import assert_array_equal
+from functools import partial
 
 import zarr
 from zarr.core import Array
@@ -43,9 +44,16 @@ def dataset(tmpdir, request):
         if not static.exists():  # pragma: no cover
 
             if "nested" in which:
+                # No way to reproduce the nested_legacy file via code
                 generator = NestedDirectoryStore
             else:
-                generator = DirectoryStore
+                if "legacy" in suffix:
+                    # No dimension_separator metadata included
+                    generator = DirectoryStore
+                else:
+                    # Explicit dimension_separator metadata included
+                    generator = partial(DirectoryStore,
+                                        dimension_separator=".")
 
             # store the data - should be one-time operation
             s = generator(str(static))