Make it easier for Node.js to ship libuv in its tarballs without also
including the test suite. Node.js already does so but recent changes
to its build system complicate that.
Kills two birds with one stone: it helps out Node.js and it makes it
harder for us to introduce hidden dependencies between the library and
the test suite.
PR-URL: https://github.com/libuv/libuv/pull/1725
Refs: https://github.com/nodejs/node/pull/18287
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>