gdb/testsuite: unset XDG_CONFIG_HOME

Since this commit:

  commit 64aaad6349d2b2c45063a5383f877ce9a3a0c354
  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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2021-01-28  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* lib/gdb.exp (default_gdb_init): Unset XDG_CONFIG_HOME.
+
 2021-01-28  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.ada/out_of_line_in_inlined.exp: Use gdb_breakpoint.

gdb/testsuite/lib/gdb.exp

@@ -5241,6 +5241,13 @@ proc default_gdb_init { test_file_name } {
     unset -nocomplain ::env(GDBHISTFILE)
     unset -nocomplain ::env(GDBHISTSIZE)
 
+    # Ensure that XDG_CONFIG_HOME is not set.  Some tests setup a fake
+    # home directory in order to test loading settings from gdbinit.
+    # If XDG_CONFIG_HOME is set then GDB will load a gdbinit from
+    # there (if one is present) rather than the home directory setup
+    # in the test.
+    unset -nocomplain ::env(XDG_CONFIG_HOME)
+
     # Initialize GDB's pty with a fixed size, to make sure we avoid pagination
     # during startup.  See "man expect" for details about stty_init.
     global stty_init