Do not remove $systest for out-of-tree builds

Do not remove $systest for out-of-tree builds

Previously, the $systest directory was being removed for out-of-tree
builds at the end of each system test.  Because of that, running tests
which depend on compiled objects was breaking subsequent "make check"
invocations:

    make: Target 'check' not remade because of errors.
    Making all in dyndb/driver
    /bin/bash: line 20: cd: dyndb/driver: No such file or directory
    Making all in dlzexternal/driver
    /bin/bash: line 20: cd: dlzexternal/driver: No such file or directory

Address by first removing build/test artifacts for a given test and then
removing empty directories inside (and potentially including) $systest.