Remove dependency on Node.js headers from tests

[enlight]

node-gyp is currently used to build the test target executable, that executable has no dependencies, but node-gyp is actually meant for building native Node.js modules so it expects to have access to the Node.js headers. The only file node-gyp actually needs from Node.js is common.gypi, much of which sets things up for building native Node.js modules - stuff that is irrelevant for building the test target executable.

To remove the need to install the Node.js headers extract the useful bits from common.gypi into an identically named file and put it in this repository, then use the nodedir setting with node-gyp configure to point node-gyp to the directory containing the slimmed down common.gypi.