tests: use filepaths directly

After a long IRC discussion, it was decided that the use of direct
filepaths instead of the module is a more pure form of testing ,
although it may provide less overall coverage than the mixed integration
style imports used previously.

This will need to be remedied by further integration testing in
/test/integration.

test/bufferutils.js

@@ -1,5 +1,5 @@
 var assert = require('assert')
-var bufferutils = require('..').bufferutils
+var bufferutils = require('../src/bufferutils')
 
 var fixtures = require('./fixtures/buffer.js')