Further tests for structured arrays

[alimanfoo]

More tests are needed for basic functionality for arrays with structured dtype, including fill value, 0d array.

[jakirkham]

Out of curiosity, are there existing tests for structured arrays somewhere?

[jakirkham]

Looks like there were some structured array tests including with fill value already. However there wasn't a test for 0-D structured arrays. So tweaked the existing tests to include the 0-D case. Put this together as PR ( https://github.com/alimanfoo/zarr/pull/210 ).