Make all unignored zarr warnings errors (#1021)

Following on from #654, I wanted to make sure that
no warnings are being missed by the tests.

Author: joshmoore

pytest.ini

@@ -2,7 +2,7 @@
 doctest_optionflags = NORMALIZE_WHITESPACE ELLIPSIS IGNORE_EXCEPTION_DETAIL
 addopts = --durations=10
 filterwarnings =
-    error::DeprecationWarning:zarr.*
-    error::UserWarning:zarr.*
+    error:::zarr.*
+    ignore:Not all N5 implementations support blosc compression.*:RuntimeWarning
     ignore:PY_SSIZE_T_CLEAN will be required.*:DeprecationWarning
     ignore:The loop argument is deprecated since Python 3.8.*:DeprecationWarning