test_dim_separator.py Generate fixtures when needed (#824)

* test_dim_separator.py Generate fixtures when needed

see: conda-forge/zarr-feedstock#50

* Correct flake8 issues

* Add 'pragma: no cover' to the one-off

zarr/tests/test_dim_separator.py

@@ -36,9 +36,19 @@ def dataset(tmpdir, request):
     if which.startswith("static"):
         project_root = pathlib.Path(zarr.__file__).resolve().parent.parent
         if which.endswith("nested"):
-            return str(project_root / "fixture/nested")
+            static = project_root / "fixture/nested"
+            generator = NestedDirectoryStore
         else:
-            return str(project_root / "fixture/flat")
+            static = project_root / "fixture/flat"
+            generator = DirectoryStore
+
+        if not static.exists():  # pragma: no cover
+            # store the data - should be one-time operation
+            s = generator(str(static))
+            a = zarr.open(store=s, mode="w", shape=(2, 2), dtype="<i8")
+            a[:] = [[1, 2], [3, 4]]
+
+        return str(static)
 
     if which.startswith("directory"):
         store_class = DirectoryStore