cooked_read test for readonly regcache

This patch adds a test to check cooked_read for readonly regcache.  For
raw registers, cooked_read get either REG_VALID or REG_UNKNOWN, depends on
the raw register is in save_reggroup or not.  For pseudo register,
cooked_read get different result in different ports.

gdb:

2017-11-24  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (cooked_read_test): Add more test for readonly
	regcache.