Run valgrind on early psets to test for errors that may be masked

[curiouskiwi]

As an example, an uninitialised value (causing undefined behaviour) can be masked by the -fsanitize clang flag, so the program works in the IDE but not in check50. Running valgrind would find this error, so we could return a better error to in the check50 results.

[cmlsharp]

Obviously this functionality exists in check50 since we use this for speller, however the message that is currently printed in the log is checking for valgrind errors.... We should likely change this message if we are going to use this on earlier psets. Thoughts on what the message should be? Perhaps "checking for undefined behavior and memory leaks", or similar?