test: add parallel test runs on macOS and Windows
test: add parallel test runs on macOS and Windows

Note that the Windows run is in __allow_failures__ mode but that should be
fixed in a separate PR.

PR-URL: https://github.com/nodejs/node-gyp/pull/1800
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp