Test user-defined gdb commands and arguments stack

Test user-defined gdb commands and arguments stack

We're missing a test that makes sure that arguments to user-defined
commands are handled correctly when a user-defined command calls
another user-defined command / recurses.

The following patch changes that code, so add such a test first so we
can be confident won't be breaking this use case.

gdb/testsuite/ChangeLog:
2016-12-02  Pedro Alves  <palves@redhat.com>

	* gdb.base/commands.exp (user_defined_command_args_stack_test):
	New procedure.
	(top level): Call it.