test: fix warning in test_environment.cc

warning log:

../test/cctest/test_environment.cc: In constructor ‘RedirectStdErr::RedirectStdErr(const char*)’:
../test/cctest/test_environment.cc:77:12: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result]
     freopen(filename_, "w", stderr);
     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~