[committed][gdb/testsuite] Disable vgdb tests if xml not supported

[Tom Tromey]

Tom> Or perhaps you mean more generically (which wouldn't require us to write
Tom> a proc for each type of require):
Tom> ...
Tom> proc require { fn val } {
Tom>     if { [$fn] != $val } {
Tom>         untested "$fn != $val"
Tom>         return -code return 0
Tom>     }
Tom> }
Tom> ...
Tom> and:
Tom> ...
Tom> require gdb_skip_xml_test 0
Tom> ...
Tom> ?

Either seems fine but this one does read nicely to me.
Or maybe even just requiring 'fn' to return 0 would be good enough.

Tom