Test speed ups in test_distarray by reducing roundtrips.

speed things up by replacing for loops over arrays with calls to
.toarray(). Which still tests that the array can be indexed, but it
pulls it to the client and does it all on the client instead of
remotely.

This change affects:
    * test_from_global_dim_data_irregular_block
    * test_from_global_dim_data_bu
    * test_from_global_dim_data_uu
    * test_from_global_dim_data_bc
    * test_irregular_block_assignment