fpi-device: Only use emulation mode by pre-loading a library

Rather than just do it in production if set, make it more difficult to
potentially inject the test mode.

So rely on a weak definition of fpi_device_emulation_mode_enabled() that
can be only set externally

tests/test-emulation.c

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2026 Canonical, Ltd.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "fpi-device.h"
+
+gboolean
+  (fpi_device_emulation_mode_enabled) (FpDevice *device)
+{
+  static gsize emulation_mode = 0;
+
+  if (g_once_init_enter (&emulation_mode))
+    g_once_init_leave (&emulation_mode,
+                       g_strcmp0 (g_getenv ("FP_DEVICE_EMULATION"), "1") == 0 ?
+                       TRUE : G_MAXSIZE);
+
+  return emulation_mode == TRUE;
+}