linux: cache netmask for accurate subnet prefix calculation to fix implementation which always returned 0 (#1155)

Co-authored-by: Syed Amer Gilani <syed.gilani@trilux.com>

test/ports/linux/bip_subnet/src/main.c

@@ -0,0 +1,49 @@
+/**
+ * @file
+ * @brief Tests for BACnet/IP subnet prefix caching on Linux
+ * @copyright SPDX-License-Identifier: MIT
+ */
+#include <zephyr/ztest.h>
+#include <bacnet/datalink/bip.h>
+
+static void test_prefix_defaults_to_zero(void)
+{
+    bip_cleanup();
+    zassert_equal(
+        bip_get_subnet_prefix(), 0, "Prefix should be zero by default");
+}
+
+static void test_prefix_roundtrip(void)
+{
+    bip_cleanup();
+    zassert_true(bip_set_subnet_prefix(24), NULL);
+    zassert_equal(bip_get_subnet_prefix(), 24, NULL);
+
+    zassert_true(bip_set_subnet_prefix(16), NULL);
+    zassert_equal(bip_get_subnet_prefix(), 16, NULL);
+}
+
+static void test_prefix_invalid_values(void)
+{
+    bip_cleanup();
+    zassert_false(bip_set_subnet_prefix(0), NULL);
+    zassert_false(bip_set_subnet_prefix(33), NULL);
+    zassert_equal(bip_get_subnet_prefix(), 0, NULL);
+}
+
+static void test_prefix_all_bits_set(void)
+{
+    bip_cleanup();
+    zassert_true(bip_set_subnet_prefix(32), NULL);
+    zassert_equal(bip_get_subnet_prefix(), 32, NULL);
+}
+
+void test_main(void)
+{
+    ztest_test_suite(
+        bip_subnet_test, ztest_unit_test(test_prefix_defaults_to_zero),
+        ztest_unit_test(test_prefix_roundtrip),
+        ztest_unit_test(test_prefix_invalid_values),
+        ztest_unit_test(test_prefix_all_bits_set));
+    ztest_run_test_suite(bip_subnet_test);
+}