Add self-test framework to gdb

Add self-test framework to gdb

I wanted to unit test the Rust lexer, so I added a simple unit testing
command to gdb.

The intent is that self tests will only be compiled into gdb in
development mode.  In release mode they simply won't exist.  So, this
exposes $development to C code as GDB_SELF_TEST.

In development mode, test functions are registered with the self test
module.  A test function is just a function that does some checks, and
throws an exception on failure.

Then this adds a new "maint selftest" command which invokes the test
functions, and a new dejagnu test case that invokes it.

2016-05-17  Tom Tromey  <tom@tromey.com>

	* NEWS: Add "maint selftest" entry.
	* selftest.h: New file.
	* selftest.c: New file.
	* maint.c: Include selftest.h.
	(maintenance_selftest): New function.
	(_initialize_maint_cmds): Add "maint selftest" command.
	* configure.ac (GDB_SELF_TEST): Maybe define.
	* config.in, configure: Rebuild.
	* Makefile.in (SFILES): Add selftest.c.
	(COMMON_OBS): Add selftest.o.

2016-05-17  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Maintenance Commands): Document "maint selftest".

2016-05-17  Tom Tromey  <tom@tromey.com>

	* gdb.gdb/unittest.exp: New file.