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.