Suppress test warnings

Addresses #1182.  We need to add the following `__init__.py` to all test packages and use the `skimage._shared.utils.all_warnings` context manager for known warnings:

```
import warnings
warnings.simplefilter('error')
```

Need to implement for all modules:

[ x] color
[ ] data
[ ] exposure
[ ] external
[ ] feature
[ ] feature
[ ] filters
[ ] graph
[ ] io
[ ] measure
[ ] morphology
[ ] novice
[ ] restoration
[ ] scripts
[ ] segmentation
[ ] _shared
[ ] transform
[ ] util
[ ] viewer