regcache::cooked_write test

regcache::cooked_write test

Since my following patches will change how each gdbarch read and write
pseudo registers, it's better to write a unit test to
regcache::cooked_write, to make sure my following changes don't cause
any regressions.  See the comments on cooked_write_test.

gdb:

2018-01-22  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (cooked_write_test): New function.
	(_initialize_regcache): Register the test.