valgrind for system tests

[bbell]

In order to control memory leaks in Rasdaman, one should additionally pass system test queries in "test_select" with:

valgrind —leak-check=full directql 'query' -d DB_DIRECTORY

Ideally, one would parse the output of valgrind and use that data to determine whether or not a leak is excessive.

An initial threshold for when a leak is "too big" could be around 50kB; however, the size of the leak might be a linear function of the size of the stored data, so perhaps a few conditionals to determine pass/failure of queries would be of use here.