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.

gdb/testsuite/ChangeLog

@@ -1,4 +1,10 @@
-2015-04-07  Pedro Alves  <palves@redhat.com>
+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.
+
+2016-04-07  Pedro Alves  <palves@redhat.com>
 
 	* gdb.compile/compile.exp: Use gdb_compile with "shlib=" option
 	instead of build_executable.  Use gdb_load_shlibs.

gdb/testsuite/gdb.base/gdbhistsize-history.exp

@@ -32,7 +32,13 @@ if { [is_remote host] } {
 proc test_histsize_history_setting { histsize size { env_var "GDBHISTSIZE" } } {
     global env
 
-    save_vars { env($env_var) } {
+    save_vars { env(GDBHISTFILE) env(GDBHISTSIZE) env($env_var) } {
+	# These environment variables take precedence over whatever
+	# history size is set in .gdbinit.  Make sure the former is not
+	# set.
+	unset -nocomplain env(GDBHISTFILE)
+	unset -nocomplain env(GDBHISTSIZE)
+
 	set env($env_var) $histsize
 
 	with_test_prefix "histsize=$histsize" {

gdb/testsuite/gdb.base/gdbinit-history.exp

@@ -36,12 +36,13 @@ proc test_gdbinit_history_setting { home size { gdbhistsize_val "-" } } {
     global srcdir
     global subdir
 
-    save_vars { INTERNAL_GDBFLAGS env(GDBHISTSIZE) env(HOME) } {
+    save_vars { INTERNAL_GDBFLAGS env(GDBHISTFILE) env(GDBHISTSIZE) env(HOME) } {
 	set env(HOME) "$srcdir/$subdir/$home"
 
-	# The GDBHISTSIZE environment variable takes precedence over whatever
+	# These environment variables take precedence over whatever
 	# history size is set in .gdbinit.  Make sure the former is not
 	# set.
+	unset -nocomplain env(GDBHISTFILE)
 	unset -nocomplain env(GDBHISTSIZE)
 
 	if { $gdbhistsize_val != "-" } {
@@ -77,10 +78,11 @@ proc test_no_truncation_of_unlimited_history_file { } {
     global env
     global INTERNAL_GDBFLAGS
 
-    save_vars { INTERNAL_GDBFLAGS env(GDBHISTSIZE) } {
-	# The GDBHISTSIZE environment variable takes precedence over whatever
+    save_vars { INTERNAL_GDBFLAGS env(GDBHISTFILE) env(GDBHISTSIZE) } {
+	# These environment variables take precedence over whatever
 	# history size is set in .gdbinit.  Make sure the former is not
 	# set.
+	unset -nocomplain env(GDBHISTFILE)
 	unset -nocomplain env(GDBHISTSIZE)
 
 	set temp_gdbinit [standard_output_file "gdbinit-history.gdbinit"]