test: improve the code in test-crypto-dh

* validate the errors for all assert.throws
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10734


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>