gdb/testsuite: add some xfail in gdb.base/startup-with-shell.exp

There are two tests that fail in gdb.base/startup-with-shell.exp when
using the native-extended-remote board.  I plan to fix these issues,
and I've posted a series that does just that:

  https://inbox.sourceware.org/gdb-patches/cover.1730731085.git.aburgess@redhat.com

But until that series is reviewed, I thought I'd merge this commit,
which marks the FAIL as XFAIL and links them to the relevant bug
number.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28392

Tested-By: Guinevere Larsen <guinevere@redhat.com>