test: use typed atomics in test files (#25071)

Use typed atomics (atomic.Int64, atomic.Int32, etc.) in test files to prevent
mixing atomic and non-atomic access on the same value, guarantee 64-bit
alignment on 32-bit platforms, and provide a cleaner API.

scaletest/agentconn/run_test.go

@@ -264,14 +264,12 @@ func setupRunnerTest(t *testing.T) (client *codersdk.Client, agentID uuid.UUID)
 func testServer(t *testing.T) (string, func() int64) {
 	t.Helper()
 
-	var count int64
+	var count atomic.Int64
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		atomic.AddInt64(&count, 1)
+		count.Add(1)
 		w.WriteHeader(http.StatusOK)
 	}))
 	t.Cleanup(srv.Close)
 
-	return srv.URL, func() int64 {
-		return atomic.LoadInt64(&count)
-	}
+	return srv.URL, count.Load
 }