tools: allow test.py to use full paths of tests

Allow test.py to run tests with a 'tests/' prefix or a '.js' postfix

PR-URL: https://github.com/nodejs/node/pull/9694
Fixes: https://github.com/nodejs/node/issues/9684


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>