test: suppress unused warning postmortem_metadata

[danbev]

Currently the following warning is generated by when building the
cctests:

../test/cctest/test_node_postmortem_metadata.cc:133:13:
warning: unused variable 'it' [-Wunused-variable]
  for (auto it : *(*env)->req_wrap_queue()) {}
            ^
1 warning generated.

This commmit adds a cast to void to suppres the warning.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

[danbev]

CI: https://ci.nodejs.org/job/node-test-pull-request/12923/

[cjihrig]

Duplicate of #18530?

[danbev]

@cjihrig It is, sorry about that.