Fix gdb.base/gdbinit-history.exp when HISTSIZE is set in the environment

Fix gdb.base/gdbinit-history.exp when HISTSIZE is set in the environment

Some buildslaves are showing that this test is failing.  E.g.,:

 https://sourceware.org/ml/gdb-testers/2015-q2/msg04164.html

The issue is that HISTSIZE is set to 1000 in the environment that runs
the tests (that's the default in Fedora, set in /etc/profile).

We can trivially reproduce it with:

 $ HISTSIZE=1000 make check RUNTESTFLAGS="gdbinit-history.exp"
 (...)
 Running /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/gdbinit-history.exp ...
 FAIL: gdb.base/gdbinit-history.exp: show history size
 FAIL: gdb.base/gdbinit-history.exp: show history size
 FAIL: gdb.base/gdbinit-history.exp: show commands

gdb.log shows:
 ...
 (gdb) set height 0
 (gdb) set width 0
 (gdb) show history size
 The size of the command history is 1000.
 (gdb) FAIL: gdb.base/gdbinit-history.exp: show history size

gdb/testsuite/ChangeLog:
2015-05-19  Pedro Alves  <palves@redhat.com>

	* gdb.base/gdbinit-history.exp (test_gdbinit_history_setting):
	Save the whole env array instead of just HOME.  Unset HISTSIZE in
	the environment while testing.  Restore whole environment
	afterwards.