test: fix n-api addon build warnings

Fixed an MSVC warning on Windows:
* test_general.c - Lossy conversion from int64 to double, explicitly
  casting to double resolved the warning

PR-URL: https://github.com/nodejs/node/pull/21808


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