[lldb] [test] Skip watchpoint tests on NetBSD if userdbregs is disabled

Skip watchpoint tests if security.models.extensions.user_set_dbregs
is disabled.  This indicates that unprivileged processes are not allowed
to write to debug registers which is a prerequisite for using hardware
watchpoints.

Differential Revision: https://reviews.llvm.org/D63380

llvm-svn: 363536