* entc/gen: add enttest package for generated code.
It's still WIP and another 2 things need to be added:
- Add an option to skip schema migration and pass client options
- Update docs and replace existing testing code to use enttest
* entc/gen: finish enttest package tempalte
Signed-off-by: Alex Snast <alexsn@fb.com>
Co-authored-by: Alex Snast <alexsn@fb.com>