test: fix coverity UNINIT_CTOR cctest warning

Explicitly initialize `platform_` to nullptr.  Coverity cannot divine
it is set and cleared by the Setup() and TearDown() methods.

PR-URL: https://github.com/nodejs/node/pull/12387


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>