tests/qtest: Add npcm7xx timer test

This test exercises the various modes of the npcm7xx timer. In
particular, it triggers the bug found by the fuzzer, as reported here:

https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg02992.html



It also found several other bugs, especially related to interrupt
handling.

The test exercises all the timers in all the timer modules, which
expands to 180 test cases in total.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-id: 20201008232154.94221-2-hskinnemoen@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>