Memory leak check reported as success when it was not executed

[mweisgut]

In test.sh, the memory leak check is executed with valgrind. However, we dump the output to /dev/null. Thus, even if valgrind is actually not installed and the memory check cannot be executed (valgrind: command not found is dumped to /dev/null), the resulting message is Memory leak check succeeded!.

We should report a warning / error message if the vagrind memory check cannot be executed.