* gdb.c++/inherit.exp (test_ptype_si): Fix tagless struct ptype

	* gdb.c++/inherit.exp (test_ptype_si): Fix tagless struct ptype
 	tests and anonymous union print/ptype tests.
	* gdb.base/list.exp (test_forward-search): Increase timeout by
	5 minutes for the "search extremely long line" case.
	* lib/gdb.exp (gdb_test): Document that the third arg to gdb_test is
	completely optional and that the pass/fail messages use the command as
	the message if that third arg is a null string.
	(gdb_test_exact):  Arrange that a null string pattern means match a
	null string output rather than any output, which might include random
 	errors.
	* gdb.base/mips_pro.exp: Add "mips*-sgi-irix4*" xfail for
	backtrace test.
	* gdb.c++/demangle.exp: Add a "*-*-*" xfail for test that was
	always failing and failure is now exposed by gdb.exp changes.