test(integration): add throwing and counter web fixtures

internal/integration/testdata/counter/index.html

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+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>counter</title>
+  </head>
+  <body>
+    <button id="decrement">-</button>
+    <div id="counter">0</div>
+    <button id="increment">+</button>
+    <script>
+      let value = 0;
+      const display = document.getElementById("counter");
+      function render() {
+        display.textContent = String(value);
+      }
+      document.getElementById("increment").addEventListener("click", function () {
+        value += 1;
+        render();
+      });
+      document.getElementById("decrement").addEventListener("click", function () {
+        value -= 1;
+        render();
+      });
+    </script>
+  </body>
+</html>

internal/integration/testdata/throwing/index.html

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+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>throwing</title>
+  </head>
+  <body>
+    <button id="press">press</button>
+    <div id="count">0</div>
+    <script>
+      let presses = 0;
+      document.getElementById("press").addEventListener("click", function () {
+        presses += 1;
+        document.getElementById("count").textContent = String(presses);
+        if (presses >= 3) {
+          throw new Error("pressed too many times");
+        }
+      });
+    </script>
+  </body>
+</html>