Adjust tester so doesn't report spurious "malloc shouldn't have worked" error (a result of increasing the default memory allocation).
Add filtering of program output and stderr to replace control characters (except newline and return) and values over \x7F with hexadecimal string representations. This allows output from programs writing binary junk to be correctly json-encoded but also means that program writing utf-8 output can't be run. [Actually they can't anyway because the Runguard sandbox currently runs in C locale, but that's a separate issue.]