ash testsuite: add return_in_trap1.tests

Currently it fails

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

shell/hush_test/hush-signals/return_in_trap1.right

@@ -0,0 +1,4 @@
+a:2
+b:0
+Trap
+d:3

shell/hush_test/hush-signals/return_in_trap1.tests

@@ -0,0 +1,18 @@
+a() {
+	(exit 2)
+	echo a:$?
+	(kill -s USR1 $$; echo b:$?; exit 3)
+	echo c:$?	# does not execute
+	(exit 4)
+}
+
+trap "echo Trap; return" USR1
+a
+
+echo d:$?
+# It's debatable what is the correct value above.
+# Does 'return' in trap sees $? == 2 or $? == 3?
+# IOW: after (kill..), does shell first wait for its completion
+# and sets $?, then checks pending signals and runs a trap handler,
+# or does it first checks pending signals and runs handler?
+# hush does the former, and prints 3.