test(e2e): add resolver module e2e test suite

Verifies that a configured resolver drives the proxy's outbound DNS
resolution. A gost HTTP proxy uses a resolver whose only nameserver is a
test responder answering echo.test with the real echo server IP and
NXDOMAIN for everything else. A request to echo.test is resolved by the
custom resolver and reaches the echo server; an unmapped host fails to
resolve. Adds a parametrized DNS responder script and container helper.

tests/e2e/testdata/resolver/server.yaml

@@ -0,0 +1,19 @@
+# HTTP proxy whose outbound dialing uses a custom resolver. The resolver's
+# only nameserver is the test DNS responder, which answers echo.test with the
+# real echo server IP and NXDOMAIN for everything else. A request to
+# echo.test:5678 is therefore resolved by the custom resolver and reaches the
+# echo server; a request to an unmapped host fails to resolve.
+services:
+  - name: proxy
+    addr: ":8080"
+    resolver: resolver-0
+    handler:
+      type: http
+    listener:
+      type: tcp
+
+resolvers:
+  - name: resolver-0
+    nameservers:
+      - addr: dns-responder:5353
+        timeout: 3s