`match' handler now defined in terms of output abstraction

Consequently, now supports `on' clause.

src/expect-core

@@ -165,9 +165,12 @@ __expect--output-clause()
 # in regex(3))
 _expect--match()
 {
-  shift "$2"
-  local -r pat="$1"
+  __expect--output-cmd '__expect--match-do' "$@"
+}
 
+__expect--match-do()
+{
+  local -r pat="$1"
   [[ "$(cat)" =~ $pat ]]
 }
 

test/test-expect-core

@@ -119,6 +119,14 @@ describe match
       to succeed
   end
 
+  # by convention, stderr file passed as third argument
+  it can assert against stderr output
+    {
+      expect _expect--match 0 3 /dev/fd/3 "stderr" on stderr
+        to succeed
+    } 3< <( echo "test stderr" )
+  end
+
   it ignores exit code
     expect _expect--match 1 2 "foo" <<< "foo"
       to succeed