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/.gitignore b/openbsc/.gitignore
index 04ebc2f..a3ccd45 100644
--- a/openbsc/.gitignore
+++ b/openbsc/.gitignore
@@ -54,6 +54,7 @@
 tests/gprs/gprs_test
 
 tests/atconfig
+tests/atlocal
 tests/package.m4
 tests/testsuite
 tests/testsuite.log