test: suppress unused warning postmortem_metadata

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