Fix "maint selftest" regression, add struct scoped_mock_context

This commit:

 commit 3922b302
 Author:     Pedro Alves <palves@redhat.com>
 AuthorDate: Thu Jun 18 21:28:37 2020 +0100

    Decouple inferior_ptid/inferior_thread(); dup ptids in thread list (PR 25412)

caused a regression for gdb.gdb/unittest.exp when GDB is configured
with --enable-targets=all.  The failure is:

  gdb/thread.c:95: internal-error: thread_info* inferior_thread(): Assertion `current_thread_ != nullptr' failed.

The problem is in this line in regcache.c:cooked_read_test:

  /* Switch to the mock thread.  */
  scoped_restore restore_inferior_ptid
    = make_scoped_restore (&inferior_ptid, mock_ptid);

Both gdbarch-selftest.c and regcache.c set up a similar mock context,
but the series the patch above belongs to only updated the
gdbarch-selftest.c context to not write to inferior_ptid directly, and
missed updating regcache.c's.

Instead of copying the fix over to regcache.c, share the mock context
setup code in a new RAII class, based on gdbarch-selftest.c's version.

Also remove the "target already pushed" error from regcache.c, like it
had been removed from gdbarch-selftest.c in the multi-target series.
That check is unnecessary because each inferior now has its own target
stack, and the unit test pushes a target on a separate (mock)
inferior, not the current inferior on entry.

gdb/ChangeLog:
2020-06-23  Pedro Alves  <palves@redhat.com>

	* gdbarch-selftests.c: Don't include inferior.h, gdbthread.h or
	progspace-and-thread.h.  Include scoped-mock-context.h instead.
	(register_to_value_test): Use scoped_mock_context.
	* regcache.c: Include "scoped-mock-context.h".
	(cooked_read_test): Don't error out if a target is already pushed.
	Use scoped_mock_context.  Adjust.
	* scoped-mock-context.h: New file.