test: Add bats-core test framework

- Create test_helper/common.bash with shared utilities
- Add unit tests for firewall configuration functions
- Add unit tests for build script functions
- Establish testing patterns for TDD approach

This provides the foundation for 100% test coverage.

💘 Generated with Crush

Assisted-by: GLM-4.6 via Crush <crush@charm.land>

tests/test_helper/common.bash

@@ -0,0 +1,38 @@
+#!/usr/bin/env bats
+# Test helper setup for bats-core
+
+# Load bats support libraries if available
+if [[ -f "/usr/lib/bats-core/bats-support/load.bash" ]]; then
+    load '/usr/lib/bats-core/bats-support/load'
+    load '/usr/lib/bats-core/bats-assert/load'
+    load '/usr/lib/bats-core/bats-file/load'
+fi
+
+# Common test variables
+readonly TEST_TEMP_DIR=$(mktemp -d)
+readonly PROJECT_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+# Cleanup function
+cleanup() {
+    rm -rf "$TEST_TEMP_DIR"
+}
+
+# Set up trap for cleanup
+trap cleanup EXIT
+
+# Common helper functions
+create_test_wg_config() {
+    local config_file="$1"
+    cat > "$config_file" << EOF
+[Interface]
+PrivateKey = testPrivateKey1234567890abcdefghijklmnopqrstuvwxyz
+Address = 10.0.0.2/24
+DNS = 1.1.1.1
+
+[Peer]
+PublicKey = testPublicKey1234567890abcdefghijklmnopqrstuvwxyz
+Endpoint = 192.168.1.100:51820
+AllowedIPs = 0.0.0.0/0
+PersistentKeepalive = 25
+EOF
+}