Refactor Unbound setup and add E2E tests (#1340)

Refactor Unbound DNS installation to use modern `conf.d` pattern and add
E2E testing.

**Changes:**
- Unified Unbound config across all distros using
`/etc/unbound/unbound.conf.d/openvpn.conf`
- Added startup validation with retry logic
- Added `ip-freebind` to allow binding before tun interface exists
- E2E tests now verify Unbound DNS resolution from VPN clients

**Testing:**
- Server: verifies config creation, interface binding, security options
- Client: verifies DNS resolution through Unbound (10.8.0.1)

---

Closes https://github.com/angristan/openvpn-install/issues/602 Closes
https://github.com/angristan/openvpn-install/pull/604 Closes
https://github.com/angristan/openvpn-install/issues/1189

Co-authored-by: Henry N <henrynmail-github@yahoo.de>

test/Dockerfile.server

@@ -8,21 +8,22 @@ ARG BASE_IMAGE
 ENV DEBIAN_FRONTEND=noninteractive
 
 # Install basic dependencies based on the OS
+# dnsutils/bind-utils provides dig for DNS testing with Unbound
 RUN if command -v apt-get >/dev/null; then \
         apt-get update && apt-get install -y --no-install-recommends \
-            iproute2 iptables curl procps systemd systemd-sysv \
+            iproute2 iptables curl procps systemd systemd-sysv dnsutils \
         && rm -rf /var/lib/apt/lists/*; \
     elif command -v dnf >/dev/null; then \
         dnf install -y --allowerasing \
-            iproute iptables curl procps-ng systemd tar gzip \
+            iproute iptables curl procps-ng systemd tar gzip bind-utils \
         && dnf clean all; \
     elif command -v yum >/dev/null; then \
         yum install -y \
-            iproute iptables curl procps-ng systemd tar gzip \
+            iproute iptables curl procps-ng systemd tar gzip bind-utils \
         && yum clean all; \
     elif command -v pacman >/dev/null; then \
         pacman -Syu --noconfirm \
-            iproute2 iptables curl procps-ng \
+            iproute2 iptables curl procps-ng bind \
         && pacman -Scc --noconfirm; \
     fi