test: Update test suite with improved structure

- Update test_helper/common.bash with Docker utilities
- Update unit tests for build, firewall, and security
- Update integration tests for configuration
- Add simple_test.bats for basic testing
- Fix test assertions and error handling

💘 Generated with Crush

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

tests/test_helper/common.bash

@@ -3,9 +3,9 @@
 
 # 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'
+    bats_load_library "/usr/lib/bats-core/bats-support"
+    bats_load_library "/usr/lib/bats-core/bats-assert"
+    bats_load_library "/usr/lib/bats-core/bats-file"
 fi
 
 # Common test variables
@@ -35,4 +35,32 @@ Endpoint = 192.168.1.100:51820
 AllowedIPs = 0.0.0.0/0
 PersistentKeepalive = 25
 EOF
+}
+
+# Additional helper functions for missing assertions
+assert_file_exists() {
+    if [[ ! -f "$1" ]]; then
+        echo "File does not exist: $1"
+        return 1
+    fi
+}
+
+assert_file_contains() {
+    local file="$1"
+    local content="$2"
+    
+    if ! grep -q "$content" "$file"; then
+        echo "File '$file' does not contain '$content'"
+        return 1
+    fi
+}
+
+assert_regex() {
+    local haystack="$1"
+    local pattern="$2"
+    
+    if ! echo "$haystack" | grep -qE "$pattern"; then
+        echo "Output does not match regex pattern '$pattern'"
+        return 1
+    fi
 }