Really stop affecting user command history (re: aff63e38)

The fix was incomplete because some tests have to unset HISTFILE,
which reverted them to using ~/.sh_history by default.

src/cmd/ksh93/tests/shtests:
- Instead of setting HISTFILE, set HOME to the temporary directory
  $tmp, so nothing will write to the real user directory and the
  default history file is $tmp/.sh_history.

src/cmd/ksh93/tests/attributes.sh:
- Restore HISTFILE after a test that requires setting HISTFILE=foo.

src/cmd/ksh93/tests/shtests

@@ -298,7 +298,8 @@ else	trap 'cd / && rm -rf "$tmp"' EXIT
 fi
 
 # for interactive shell ('ksh -i') tests: avoid affecting ~/.sh_history
-export HISTFILE=$tmp/sh_history
+# (some tests must unset or modify $HISTFILE, so set $HOME instead)
+export HOME=$tmp
 
 if	(( compile ))
 then	if	whence $SHCOMP > /dev/null