Don't write to inferior_ptid in gdbarch-selftests.c, mock address_space too

Don't write to inferior_ptid in gdbarch-selftests.c, mock address_space too

Use switch_to_thread instead of writing to inferior_ptid.  This
requires a couple of improvements to the mocking environment.  One is
to mock a pspace too, and assigning it to the inferior.  In turn, this
requires heap-allocating the address space, so that the regular
program_space dtor destroys the address space correctly.

(Note that new the mock program_space is allocated on the stack, and
thus depends on the previous patch that eliminated
delete_program_space.)

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* gdbarch-selftests.c: Include "progspace-and-thread.h".
	(register_to_value_test): Mock a program_space too.  Heap-allocate
	the address space.  Don't write to inferior_ptid.  Use
	switch_to_thread instead.