nat: Fix some memory leaks in the testcases.

The test cases did not free some of the resources it allocated.
Improve the situation a bit. There is still data allocated that
is not freed at exit.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index b035004..2b30413 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -452,6 +452,8 @@
 		printf("Should have found it.\n");
 		abort();
 	}
+
+	talloc_free(nat);
 }
 
 static void test_mgcp_allocations(void)
@@ -820,6 +822,7 @@
 	}
 
 	msgb_free(msg);
+	talloc_free(nat);
 }
 
 static void test_dt_filter()
@@ -1040,6 +1043,8 @@
 	verify_msg(out, cc_setup_national_again,
 			ARRAY_SIZE(cc_setup_national_again));
 	msgb_free(out);
+	bsc_nat_num_rewr_entry_adapt(nat, &nat->num_rewr, NULL);
+	talloc_free(nat);
 }
 
 static void test_setup_rewrite_prefix(void)