gdb.gdbtk tests

[Jimmy Guo]

These tests apparently require top level tk/, tix/, itcl/, and libgui/
(the Cygnus GUI) components be present in the source tree (per
gdb/testsuite/lib/gdb.exp (gdbtk_start)).  Will these components be part
of public tree any time soon?

Besides, it's better to have a sanity check routine testing all
dependencies before even trying gdb_compile, like the skip_hp_tests one.

Some codes like below will be better:

set path [file join <whatever path>]
if ![file isdirectory $path] {
   perror <error message>
   return 1 (or exit 1)
}

- Jimmy Guo, guo@cup.hp.com