[test] Fix `shell` not found error

When I run `git commit` in my iTerm, I get the following error message from the precommit hook:
```
> memcached@0.2.8 test /Users/Fiona/workspace/node-memcached
> mocha $(shell find test -name '*.test.js')

sh: shell: command not found
```

This PRs uses a regexp to find the test files instead of calling out to `shell find`.