Fix "shared example group previously defined" warning

We were getting the warning

```
WARNING: Shared example group 'crop suggest' has been previously defined
at:
  /Users/miles/src/growstuff/spec/features/shared_examples/crop_suggest_spec.rb:3
...and you are now defining it at:
  /Users/miles/src/growstuff/spec/features/shared_examples/crop_suggest_spec.rb:3
The new definition will overwrite the original one.
```

Following the suggestion at
https://github.com/rspec/rspec-core/issues/828#issuecomment-38789977,
I've renamed crop_suggest_spec.rb to crop_suggest.rb, which made the
error go away without reducing the number of tests run. RSpec must
have thought it was a spec file and loaded it directly, then loaded it
again when it was first required by an actual spec file.