build: don't fail `make test` on source tarballs

Tries to achieve the same effect as
https://github.com/nodejs/node/pull/13658 without breaking source
tarballs. Presumably if `tools/eslint` wasn't there at all, people
would notice in the PR review!

PR-URL: https://github.com/nodejs/node/pull/15441
Fixes: https://github.com/nodejs/node/issues/14513


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>