gdb: refactor test_get_thread_arch_aspace_regcache

gdb: refactor test_get_thread_arch_aspace_regcache

Do these misc changes to test_get_thread_arch_aspace_regcache:

- Rename to get_thread_arch_aspace_regcache_and_check.  The following
  patch introduces a selftest for get_thread_arch_aspace_regcache, named
  get_thread_arch_aspace_regcache_test.  To avoid confusion between the
  two functions, rename this one to
  get_thread_arch_aspace_regcache_and_check, I think it describes better
  what it does.

- Remove gdbarch parameter.  We always pass the same gdbarch (the
  current inferior's gdbarch), so having a parameter is not useful.  It
  would be interesting to actually test with multiple gdbarches, to
  verify that the regcache container can hold multiple regcaches (with
  different architectures) for a same (target, ptid).  But it's not the
  case as of this patch.

- Verify that the regcache's arch is correctly set.

- Remove the aspace parameter.  We always pass NULL here, so it's not
  useful to have it as a parameter.  Also, instead of passing a NULL
  aspace to get_thread_arch_aspace_regcache and verifying that we get a
  NULL aspace back, pass the current inferior's aspace (just like we use
  the current inferior's gdbarch).

gdb/ChangeLog:

	* regcache.c (test_get_thread_arch_aspace_regcache): Rename to...
	(get_thread_arch_aspace_regcache_and_check): ... this.  Remove
	gdbarch and aspace parameter.  Use current inferior's aspace.
	Validate regcache's arch value.
	(regcaches_test): Update.

Change-Id: I8b4c2303b4f91f062269043d1f7abe1650232010