[gdb/testsuite] Fix timeout in gdb.tui/corefile-run.exp

[gdb/testsuite] Fix timeout in gdb.tui/corefile-run.exp

With test-case gdb.tui/corefile-run.exp we run into:
...
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/corefile-run.exp: load corefile
...

The timeout happens in this command:
...
Term::command "core-file $core"
...
because it tries to match "(gdb) $cmd" but $cmd is split over two lines:
...
   16 (gdb) core-file /data/vries/gdb/leap-15-4/build/gdb/testsuite/outputs/gdb.tui/co
   17 refile-run/corefile-run.core
   18 [New LWP 5370]
   19 Core was generated by `/data/vries/gdb/leap-15-4/build/gdb/testsuite/outputs/gdb
   20 .tui/corefile-run/coref'.
   21 Program terminated with signal SIGTRAP, Trace/breakpoint trap.
   22 #0  main () at tui-layout.c:21
   23 (gdb)
...

Fix this by using send_gdb "$cmd\n" and wait_for "Program terminated" instead.

Tested on x86_64-linux.