gdb/testsuite/

gdb/testsuite/
	* 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.