sh: nullify ENV in tests

This is to avoid loading .shrc which may contain commands that would
result in output different than expected.

Reviewed by:	jilles
Differential Revision:	https://reviews.freebsd.org/D35876

bin/sh/tests/builtins/cd10.0

@@ -3,4 +3,4 @@
 # Precondition
 (cd /bin) || exit
 # Verify write error is ignored.
-$SH +m -ic 'CDPATH=/:; cd bin 1</dev/null'
+ENV= $SH +m -ic 'CDPATH=/:; cd bin 1</dev/null'

bin/sh/tests/builtins/fc1.0

@@ -8,7 +8,7 @@ T=$(mktemp -d sh-test.XXXXXX)
 cd $T
 
 mkfifo input output error
-HISTFILE=/dev/null ${SH} +m -i <input >output 2>error &
+ENV= HISTFILE=/dev/null ${SH} +m -i <input >output 2>error &
 {
 	# Syntax error
 	echo ')' >&3

bin/sh/tests/execution/int-cmd1.0

@@ -1,3 +1,3 @@
 # $FreeBSD$
 
-! echo echo bad | $SH -ic 'fi' 2>/dev/null
+! echo echo bad | ENV= $SH -ic 'fi' 2>/dev/null