[gdb/testsuite] Fix gdb.base/list-dot-nodebug.exp on openSUSE

On openSUSE Leap 15.6 with test-case gdb.base/list-dot-nodebug.exp I run into:
...
(gdb) list .^M
warning: 1      ../sysdeps/x86_64/crtn.S: No such file or directory^M
(gdb) FAIL: $exp: debug=none: print before start
...

The intent of the debug=none case is to generate an executable with no debug
info.  However, we have quite a few CUs with debug info:
...
$ readelf -wi outputs/gdb.base/list-dot-nodebug/list-dot-nodebug-none \
    | egrep -c " @ "
431
...

This is because this code:
...
  gdb_gnu_strip_debug $executable no-debuglink
...
uses $executable, and the variable is set here:
...
	set executable ${testfile}-none
...
which sets it to "list-dot-nodebug-none" and consequently
gdb_gnu_strip_debug cannot find it.

Fix this by using "[standard_output_file $executable]" instead.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/31721
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31721

gdb/testsuite/gdb.base/list-dot-nodebug.exp

@@ -55,7 +55,7 @@ foreach_with_prefix debug {"none" "some"} {
 
 	# Stripping is a backup in case the system has static libc debuginfo.
 	# We can continue the test even if it fails.
-	gdb_gnu_strip_debug $executable no-debuglink
+	gdb_gnu_strip_debug [standard_output_file $executable] no-debuglink
 
 	clean_restart ${executable}