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

When running test-case gdb.tui/corefile-run.exp on openSUSE Tumbleweed,
I run into:
...
PASS: gdb.tui/corefile-run.exp: load corefile
FAIL: gdb.tui/corefile-run.exp: run until the end
...

What's going on is easier to see when also doing dump_screen if
check_contents passes, and inspecting state at the preceding PASS:
...
 +-------------------------------------------------------------------------+
 exec No process In:                                           L??   PC: ??
 [New LWP 16629]
 [Thread debugging using libthread_db enabled]
 Using host libthread_db library "/lib64/libthread_db.so.1".
 Core was generated by `/data/gdb_versions/devel/build/gdb/testsuite/output
 s/gdb.tui/corefile-run/corefi'.
 Program terminated with signal SIGTRAP, Trace/breakpoint trap.
 #0  main ()
 --Type <RET> for more, q to quit, c to continue without paging--
...

The problem is that we're getting a pagination prompt, and the subsequent run
command is interpreted as an answer to that prompt.

Fix this by:
- detecting the gdb prompt in response to "load corefile", such that
  we detect the failure earlier, and
- doing a "set pagination off" in Term::clean_restart.

Tested on x86_64-linux.