tests/qtest/npcm7xx_pwm-test: Be less verbose unless V=2

tests/qtest/npcm7xx_pwm-test: Be less verbose unless V=2

The npcm7xx_pwm-test produces a lot of output at V=1, which
means that on our CI tests the log files exceed the gitlab
500KB limit. Suppress the messages about exactly what is
being tested unless at V=2 and above.

This follows the pattern we use with qom-test.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230209135047.1753081-1-peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>