test: fix build warning in addons-napi\test_object

Fix a build warning in the test, reported at https://github.com/nodejs/node/pull/13250#issuecomment-305670009

Note this is plain C code, so C++ style casts cannot be used.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test