[gdb/testsuite] Remove path names from error messages in gdb_file_cmd

[gdb/testsuite] Remove path names from error messages in gdb_file_cmd

In gdb_file_cmd, perror is called with error message strings using $arg and
$GDB, both of which contain path names, which makes comparison of gdb.sum
files more difficult.

Fix this by using:
- [file tail $arg] instead of $arg
- GDB instead of $GDB.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-04  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_file_cmd): Avoid path names in error messages.