tests: Use atlocal/atlocal.in to enable/disable the NAT test

Kill the hacking, use atlocal to remember if the NAT test should
be enabled and then skip the test (exit with 77).
diff --git a/openbsc/tests/atlocal.in b/openbsc/tests/atlocal.in
new file mode 100644
index 0000000..0ce168b
--- /dev/null
+++ b/openbsc/tests/atlocal.in
@@ -0,0 +1 @@
+enable_nat_test='@osmo_ac_build_nat@'