add tests for basic fd open/close/dupe

shell/hush_test/hush-misc/exec.right

@@ -0,0 +1,6 @@
+pass fd out open
+pass fd out dup
+pass fd out close
+pass fd in open
+pass fd in dup
+pass fd in close

shell/hush_test/hush-misc/exec.tests

@@ -0,0 +1,30 @@
+# make sure we have a way of checking these things
+[ ! -e /dev/fd ] && exit 1
+
+[ -e /dev/fd/44 ] && exit 1
+exec 44>/dev/null
+[ -e /dev/fd/44 ] || exit 1
+echo pass fd out open
+
+[ -e /dev/fd/55 ] && exit 1
+exec 55>&44
+[ -e /dev/fd/55 ] || exit 1
+echo pass fd out dup
+
+exec 44>&-
+[ -e /dev/fd/44 ] && exit 1
+echo pass fd out close
+
+[ -e /dev/fd/66 ] && exit 1
+exec 66</dev/null
+[ -e /dev/fd/66 ] || exit 1
+echo pass fd in open
+
+[ -e /dev/fd/77 ] && exit 1
+exec 77<&66
+[ -e /dev/fd/77 ] || exit 1
+echo pass fd in dup
+
+exec 66<&-
+[ -e /dev/fd/66 ] && exit 1
+echo pass fd in close