build: do not clean V8 gtest directory

There is a file in there required to build V8. The rest is being ignored
by V8's .gitignore anyway.

PR-URL: https://github.com/nodejs/node/pull/8317


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>