[gdb/testsuite] Use string_to_regexp on core filename in gdb_core_cmd

[gdb/testsuite] Use string_to_regexp on core filename in gdb_core_cmd

In commit 1281424ccf "[gdb/testsuite] Fix core file load FAIL in
tls-core.exp", I've made this change:
...
-       -re ": No such file or directory.*\r\n$gdb_prompt $" {
+       -re "$core: No such file or directory.*\r\n$gdb_prompt $" {
...

However, the $core variable contains a filename which needs to be matched
as a literal string, not as a regexp.

Fix this by using string_to_regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-12  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_core_cmd): Use string_to_regexp for regexp-matching
	$core.