Stop 'ksh -i' unit tests affecting user command history

Several regression tests invoke an "interactive" shell using 'ksh
-i'. This records all the commands tested in the shell's history
file. By default, that is the user's history file, ~/.sh_history.
As ksh continuously synchronises history among instances, a ksh
user who ran the regression tests ended up with a number of
mysterious extra commands in their command history.

src/cmd/ksh93/tests/shtests:
- Before running any tests, set and export HISTFILE to a new
  history file in the temporary files directory.

src/cmd/ksh93/tests/shtests

@@ -293,6 +293,9 @@ then	trap 'printf "\nTemporary files left in: %s\n" "$tmp"' EXIT
 else	trap 'cd / && rm -rf "$tmp"' EXIT
 fi
 
+# for interactive shell ('ksh -i') tests: avoid affecting ~/.sh_history
+export HISTFILE=$tmp/sh_history
+
 if	(( compile ))
 then	if	whence $SHCOMP > /dev/null
 	then	: