Testsuite: T9214: fix GRUB auto-boot race in console-select navigation

Commit 2d525e9 ("Testsuite: T9099: harden GRUB console-select navigation
with expect() check") added expect() checks after each GRUB submenu transition
in BOOTLOADERchooseSerialConsole(), but did not fix the underlying race it
described: on the installed system, BOOTLOADER_LOAD_TMO (5s) is slept before
the first KEY_DOWN is sent, while that system's top-level GRUB menu auto-boots
its default entry after the same 5s with no margin.

Under host load the countdown could win, so GRUB booted straight past
"Boot options" and the new child.expect('Select console type', ...) then
blocked for the full BOOTLOADER_TMO waiting for text that would never appear.

Send the first navigation key immediately after the GRUB banner is detected
instead of sleeping first, so the auto-boot countdown is cancelled right away
rather than raced against.

Author: c-po

scripts/check-qemu-install

@@ -595,12 +595,18 @@ def BOOTLOADERchooseSerialConsole(child, live: bool) -> None:
 
         # Wait for GRUB
         child.expect(GRUB_STRING, timeout=BOOTLOADER_TMO)
-        time.sleep(BOOTLOADER_LOAD_TMO)
 
+        # Unlike the live ISO menus (10s timeout), the installed system's
+        # top-level menu auto-boots its default entry after ~BOOTLOADER_LOAD_TMO
+        # seconds - the same delay this script would otherwise sleep before
+        # sending any key. Under host load that leaves no margin: GRUB can
+        # auto-boot before "Boot options" is ever selected. Send the first
+        # navigation key immediately to cancel the countdown, then give the
+        # screen time to settle.
         # Select GRUB serial console
         # Boot options
         child.send(KEY_DOWN)
-        time.sleep(BOOTLOADER_SLEEP)
+        time.sleep(BOOTLOADER_LOAD_TMO)
         child.send(KEY_RETURN)
         time.sleep(BOOTLOADER_SLEEP)
         # GRUB submenus never time out on their own, so confirm we actually