Handle aborted Valgrind

[broad-well]

$ runuser -u student -- valgrind --leak-check=full -s --track-origins=yes --xml=yes --xml-file=ValgrindResult.xml ./pish < /autograder/source/tests/in/test-cd-prev-basic
Current working directory: /tmp/pa4-autograder
Files: ['pish.c', 'pish_history.c', 'box', 'hello.sh', 'hello2.sh', 'script_history.sh', 'Makefile', 'pish_history.h', 'pish.o', 'pish_history.o', 'pish', 'ValgrindResult.xml']
$ runuser -u student -- valgrind --leak-check=full -s --track-origins=yes --xml=yes --xml-file=ValgrindResult.xml ./pish < /autograder/source/tests/in/test-cd-invalid
not well-formed (invalid token): line 110, column 0
stdout check: True; stderr check: True; status: True; valgrind check: False
Current working directory: /tmp/pa4-autograder
Files: ['pish.c', 'pish_history.c', 'box', 'hello.sh', 'hello2.sh', 'script_history.sh', 'Makefile', 'pish_history.h', 'pish.o', 'pish_history.o', 'pish', 'ValgrindResult.xml']
$ runuser -u student -- valgrind --leak-check=full -s --track-origins=yes --xml=yes --xml-file=ValgrindResult.xml ./pish < /autograder/source/tests/in/test-history-basic
not well-formed (invalid token): line 110, column 0

Occasionally valgrind produces invalid XML output. Currently, this causes the test to partially fail (80% of the points) but not show any valgrind problems.