test: don't call restart_uboot in EFI watchdog test

test: don't call restart_uboot in EFI watchdog test

Calling u_boot_console.restart_uboot() in
test_efi_selftest_watchdog_reboot() may lead to incorrect results.

While the watchdog triggered reboot is running thee test environment may
need some time before triggering a reboot itself. This may lead to
duplicate output of the U-Boot greeter which is recorded as an error.

Reported-by: Tom Rini <trini@konsulko.com>
Fixes: df172e117d1d ("test/py: test reboot by EFI watchdog")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>