[libcxx][test] Fix a -Wstring-concatenation warning in fuzzer_test.h

This was causing failures on green dragon:
http://lab.llvm.org:8080/green/job/libc++%20and%20libc++abi%20trunk/780/

GitOrigin-RevId: cc6d9f59fdbaf8b36b811562124ebabdc778ba2d
1 file changed