Fix warning in file_mapping_test
Fix warning in file_mapping_test

This change replaces the use of strncpy with std::copy to suppress a
-Wstringop-truncation warning. Although the strncpy usage was working as
intended, using std::copy is cleaner and removes the warning.

PiperOrigin-RevId: 224906416

google/asylo,google/asylo,google/asylo,google/asylo,google/asylo,google/asylo