gdb/testsuite: unset XDG_CONFIG_HOME

Since this commit:

  commit 64aaad63
  Date:   Fri Sep 25 14:50:56 2020 +0100

      gdb: use get_standard_config_dir when looking for .gdbinit

GDB has been checking for ${XDG_CONFIG_HOME}/gdb/gdbinit on startup.

Most tests pass -nx to GDB to block loading of gdbinit files, but
there are a few tests (e.g. gdb.base/gdbinit-history.exp) that don't
use -nx and instead setup a fake HOME directory containing a gdbinit
file.

However, since the above commit, if XDG_CONFIG_HOME is set then once
-nx is no longer being passed GDB will load any gdbinit file it finds
in that directory, which could cause the test to fail.

As a concrete example:

  $ mkdir -p fake_xdg_config_home/gdb/
  $ cat <<EOF >fake_xdg_config_home/gdb/gdbinit
  echo goodbye\n
  quit
  EOF
  $ export XDG_CONFIG_HOME=$PWD/fake_xdg_config_home
  $ make check-gdb TESTS="gdb.base/gdbinit-history.exp"

Should result in the test failing.

The solution I propose is to unset XDG_CONFIG_HOME in
default_gdb_init, we already unset a bunch of environment variables in
this proc.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (default_gdb_init): Unset XDG_CONFIG_HOME.