hush testsuite: add many tests from ash testsuite

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-signals/signal3.tests

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+$THIS_SH -c '
+hup() {
+    echo "child got HUP"
+}
+trap hup HUP
+echo "child sleeps"
+sleep 1
+echo "child exits"
+' &
+
+child=$!
+sleep 0.1 # let child install handler first
+kill -HUP $child
+wait
+echo "parent exits"