Previously, $systest directory was removed for out-of-tree builds. For
directories with compiled content (binaries), this breaks subsequent
make 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

Current approach removes only files known to Git and empty directories
in $systest (including $systest).