[gdb/testsuite] Fix gdb.tui/pr30056.exp for native-extended-gdbserver

[gdb/testsuite] Fix gdb.tui/pr30056.exp for native-extended-gdbserver

When running test-case gdb.tui/pr30056.exp with target board
native-extended-gdbserver, I run into:
...
Quit^[[K^M^[[B(gdb) PASS: gdb.tui/pr30056.exp: Control-C
Remote debugging from host ::1, port 38810^M
^M(failed reverse-i-search)`xyz': ^M(gdb) target extended-remote \
  localhost:2346^[[7GWARNING: Timed out waiting for EOF in server after \
  monitor exit
...

This is due to the fact that ^C doesn't abort the reverse-i-search.  This
appears to be due to a readline problem.  A PR is open about this: PR
cli/30498.

Add a KFAIL for the PR, and ensure that the isearch is aborted by using ^G,
such that we have a responsive prompt to handle the "monitor exit" command
that native-extended-gdbserver issues.

Tested on x86_64-linux.