[test] Fix `shell` not found error

When I run `npm run test` in my iTerm, I get the following error message:
```> memcached@0.2.8 test /Users/Fiona/workspace/node-memcached
> mocha $(shell find test -name '*.test.js')

sh: shell: command not found
```

I believe it shouldn't be necessary to additionally specify a search pattern for our test files, since they are in the default Mocha location of test/.