testsuite: Fix false FAILs with .bashrc GDBHISTFILE=...

testsuite: Fix false FAILs with .bashrc GDBHISTFILE=...

$ GDBHISTFILE=/tmp/gdbhistfile runtest gdb.base/gdbhistsize-history.exp gdb.base/gdbinit-history.exp
Running ./gdb.base/gdbinit-history.exp ...
FAIL: gdb.base/gdbinit-history.exp: home=gdbinit-history/unlimited gdbhistsize=1000: show commands
FAIL: gdb.base/gdbinit-history.exp: home=gdbinit-history/unlimited gdbhistsize=foo: show commands
Running ./gdb.base/gdbhistsize-history.exp ...
FAIL: gdb.base/gdbhistsize-history.exp: histsize=: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=20: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize= 20 : show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=-5: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=not_an_integer: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=10zab: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=-5ab: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=99999999999999999999999999999999999: show commands
FAIL: gdb.base/gdbhistsize-history.exp: histsize=50: show commands

This happens for my setup due to my:
	$ grep GDB ~/.bashrc
	export GDBHISTFILE="$HOME/.gdb_history"

gdb/testsuite/ChangeLog
2016-04-07  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/gdbhistsize-history.exp: Save and unset GDBHISTFILE and
	GDBHISTSIZE prior to the tests.
	* gdb.base/gdbinit-history.exp: Likewise.