Add a selftest that detects a 'corrupted' command tree structure in GDB.

Add a selftest that detects a 'corrupted' command tree structure in GDB.

The GDB data structure that records the GDB commands is made of
'struct cmd_list_element' defined in cli-decode.h.

A cmd_list_element has various pointers to other cmd_list_element structures,
All these pointers are together building a graph of commands.

However, when following the 'next' and '*prefixlist' pointers of
cmd_list_element, the structure must better be a tree.

If such pointers do not form a tree, then some other elements of
cmd_list_element cannot get a correct semantic.  In particular, the prefixname
has no correct meaning if the same prefix command can be reached via 2 different
paths.

This commit introduces a selftest that detects (at least some cases of) errors
leading to 'next' and '*prefixlist' not giving a tree structure.

The new 'command_structure_invariants' selftest detects one single case where
the command structure is not a tree:

  (gdb) maintenance selftest command_structure_invariants
  Running selftest command_structure_invariants.
  list 0x56362e204b98 duplicated, reachable via prefix 'show ' and 'info set '.  Duplicated list first command is 'ada'
  Self test failed: self-test failed at ../../classfix/gdb/unittests/command-def-selftests.c:160
  Ran 1 unit tests, 1 failed
  (gdb)

This was fixed by the previous commit.

2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* unittests/help-doc-selftests.c: Rename to
	unittests/command-def-selftests.c
	* unittests/command-def-selftests.c (help_doc_tests): Update some
	comments.
	(command_structure_tests, traverse_command_structure): New namespace
	and function.
	(command_structure_invariants_tests): New function.
	(_initialize_command_def_selftests) Renamed from
	_initialize_help_doc_selftests, register command_structure_invariants
	selftest.