regcache::cooked_read unit test

regcache::cooked_read unit test

This patch adds a unit test to regcache::cooked_read.  This unit test is a
little different from normal unit test, it is more about conformance test
or interaction test.  This test pass both raw register number and pseudo
register number to regcache::cooked_read, in order to inspect 1) return
value of cooked_read, 2) how are target_ops to_xfer_partial,
to_{fetch,store}_registers called (because regcache is updated by means of
these three target_ops methods).  With this test here, we have a clear
picture about how each port of GDB get cooked registers.

This patch also shares some code on mock target.

gdb:

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

	* gdbarch-selftests.c (test_target_has_registers): Move it to
	target.c.
	(test_target_has_stack): Likewise.
	(test_target_has_memory): Likewise.
	(test_target_prepare_to_store): Likewise.
	(test_target_store_registers): Likewise.
	(test_target_ops): Likewise.
	* regcache.c: Include selftest-arch.h and gdbthread.h.
	(target_ops_no_register): New class.
	(test_target_fetch_registers): New.
	(test_target_store_registers): New.
	(test_target_xfer_partial): New.
	(readwrite_regcache): New.
	(cooked_read_test): New.
	(_initialize_regcache): Register the test.
	* target.c: (test_target_has_registers): Moved from
	gdbarch-selftests.c.
	(test_target_has_stack): Likewise.
	(test_target_has_memory): Likewise.
	(test_target_prepare_to_store): Likewise.
	(test_target_store_registers): Likewise.
	* target.h (test_target_ops): New class.