test: cleanup test-intl.js

Improved variables to be all const.
Enforced assert.strictEqual across all tests.
Modified haveIntl tests to check for int.
Fixed alignment.

PR-URL: https://github.com/nodejs/node/pull/8641


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>