test(e2e): add auth module e2e test suite

Verify HTTP proxy authentication for both inline single-user auth and a
named auther with multiple users. Covers valid credentials, wrong
password, missing credentials, and any-user-in-auther acceptance.

tests/e2e/testdata/auth/inline.yaml

@@ -0,0 +1,13 @@
+# HTTP proxy with inline single-user authentication. Clients must supply the
+# proxy credentials user/pass; wrong or missing credentials get a 407 and never
+# reach the echo server.
+services:
+  - name: proxy
+    addr: ":8080"
+    handler:
+      type: http
+      auth:
+        username: user
+        password: pass
+    listener:
+      type: tcp