* gdb.base/corefile.exp: Move the core finding block out and call it as
core_find, new variable $corefile, replace corefile by $corefile and
[file tail $corefile] for usage vs. test names resp.
* lib/gdb.exp (core_find): Move it as a new function here. New
parameter binfile and deletefiles. New variable $destcore. Pre-delete
$destcore. Return "" on error.