[gdb/testsuite] Wait longer for core generation

When I run the gdb testsuite on a powerpc64le-linux system with (slow) nfs
file system, I run into timeouts due to core generation, like for instance:
...
(gdb) gcore $outputs/gdb.ada/task_switch_in_core/crash.gcore^M
FAIL: gdb.ada/task_switch_in_core.exp: save a corefile (timeout)
...

Fix this by using with_timeout_factor 3 in gdb_gcore_cmd.

Tested on powerpc64le-linux.
Approved-By: Tom Tromey <tom@tromey.com>