tests(e2e): cover the parallel selector node limit

Add an e2e case for the parallel selector configured with maxParallel: 1.
With only one node racing per connection the dead node is no longer
masked by the live one, so the requests alternate between success and
failure - that alternation is what proves the limit is applied.

Requires go-gost/x with the parallel selector node limit (gost#916).
gost's go.mod still pins v0.19.5, so this test only goes green once
that release lands and the dependency is bumped.

tests/e2e/selector_test.go

@@ -109,6 +109,28 @@ func (s *SelectorSuite) TestParallelSelector() {
 	s.Require().Contains(body, "hello-gost")
 }
 
+// TestParallelSelectorLimit exercises the parallel strategy with a node
+// limit: only one node takes part in each race, so the dead node is no
+// longer masked by the live one. With the round-robin mode the requests
+// alternate between the live node (succeeds) and the dead node (fails).
+func (s *SelectorSuite) TestParallelSelectorLimit() {
+	gostC, err := RunGostContainerWithPorts(s.ctx, SharedNetworkName, "testdata/selector/parallel_limit.yaml", "8080/tcp")
+	s.Require().NoError(err)
+	defer gostC.Terminate(s.ctx)
+
+	code, body := s.proxyRequest(gostC, "8080")
+	s.Require().Equal(0, code)
+	s.Require().Contains(body, "hello-gost", "first race should pick the live node")
+
+	code, body = s.proxyRequest(gostC, "8080")
+	s.Require().Equal(0, code)
+	s.Require().NotContains(body, "hello-gost", "second race should pick the dead node")
+
+	code, body = s.proxyRequest(gostC, "8080")
+	s.Require().Equal(0, code)
+	s.Require().Contains(body, "hello-gost", "third race should pick the live node again")
+}
+
 func TestSelectorSuite(t *testing.T) {
 	suite.Run(t, new(SelectorSuite))
 }

tests/e2e/testdata/selector/parallel_limit.yaml

@@ -0,0 +1,34 @@
+services:
+- name: proxy
+  addr: :8080
+  handler:
+    type: http
+    chain: my-chain
+  listener:
+    type: tcp
+
+- name: dummy-1
+  addr: :18081
+  handler:
+    type: http
+
+chains:
+- name: my-chain
+  hops:
+  - name: hop-1
+    selector:
+      strategy: parallel
+      # Only one node takes part in each race.
+      maxParallel: 1
+      parallelMode: round
+    nodes:
+    - name: node-1
+      addr: 127.0.0.1:18081
+      connector:
+        type: http
+    # dead node: never listens; it is no longer masked by the live node
+    # because the two of them no longer race concurrently.
+    - name: node-2
+      addr: 127.0.0.1:18082
+      connector:
+        type: http