Support -prompt and -lbl in gdb_test

Support -prompt and -lbl in gdb_test

This teaches gdb_test to forward the -prompt and -lbl options to
gdb_test_multiple.

The option parsing is done with parse_args.

As a cleanup, instead of using llength and lindex to get at the
positional arguments, use lassign, and check whether the corresponding
variable is empty.

Convert gdb.base/ui-redirect.exp and gdb.xml/tdesc-reload.exp to use
gdb_test -prompt/-lbl instead of gdb_test_multiple as examples.

Change-Id: I243e1296d32c05a421ccef30b63d43a89eaeb4a0