tests/qtest/npcm7xx_watchdog_timer: Only test the corner cases by default

tests/qtest/npcm7xx_watchdog_timer: Only test the corner cases by default

The test_prescaler() part in the npcm7xx_watchdog_timer test is quite
repetitive, testing all possible combinations of the WTCLK and WTIS
bitfields. Since each test spins up a new instance of QEMU, this is
rather an expensive test, especially on loaded host systems.
For the normal quick test mode, it should be sufficient to test the
corner settings of these fields (i.e. 0 and 3), so we can speed up
this test in the default mode quite a bit.

Message-ID: <20240115070223.30178-1-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>