build: fix test-v8 target

Clean the deps/v8 directory before rebuilding node for the hash seed
test. It is necessary to avoid the script added in [1] to use ignored
files while generating `node-debug-support.cc`.

[1]: https://github.com/nodejs/node/pull/14901

PR-URL: https://github.com/nodejs/node/pull/17269
Fixes: https://github.com/nodejs/node-v8/issues/26


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>