test: make import common as the first line

The `test/common` module has the capability to identify if any variable
is leaked to the global scope and fail the test. So that has to be
imported at the beginning.

PR-URL: https://github.com/nodejs/node/pull/7786


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>