tools,test: use Execute instead of check_output

subprocess.check_output is a python2.7 only feature. Using Execute
allows keeping python2.6 compatibility

PR-URL: https://github.com/nodejs/node/pull/17381


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>