test: add TLS performance matrix

internal/perfcheck/main_test.go

@@ -45,6 +45,18 @@ func TestExchangeRejectsInvalidResponses(t *testing.T) {
 	}
 }
 
+func TestExchangeModeTLS(t *testing.T) {
+	server := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("X-Request-ID", "perfCheck")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(expectedBody))
+	}))
+	defer server.Close()
+	if err := exchangeMode(server.Client(), strings.TrimPrefix(server.URL, "https://"), true); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestPercentileBounds(t *testing.T) {
 	var h histogram
 	// Known ranks, including a tail much slower than the median.