lib/completion-support.exp: Add test_gdb_completion_offers_commands

lib/completion-support.exp: Add test_gdb_completion_offers_commands

This adds a procedure to the collection of completion-testing
routines, that allows checking whether completion offers all commands
as completion candidates.  This will be used for testing completing
"frame apply all [TAB]", "thread apply all [TAB]", etc.

gdb/testsuite/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

        * lib/completion-support.exp (test_gdb_complete_tab_multiple)
	(test_gdb_complete_cmd_multiple, test_gdb_complete_multiple): Add
	'max_completions' parameter and handle it.
	(test_gdb_completion_offers_commands): New.