Exec module changes/fixes.

- Give Dir::monitor() a param for the polling interval, so different
  dirs can be monitored at different frequencies.

- Fix race in Exec::run() when reading extra output files produced by
  a process -- it was possible for Exec::run() to return before all
  extra output files had been fully read.

- Add test cases.

testing/scripts/httpd.py

@@ -0,0 +1,40 @@
+#! /usr/bin/env python
+
+import BaseHTTPServer
+
+class MyRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
+
+    def do_GET(self):
+        self.send_response(200)
+        self.send_header("Content-type", "text/plain")
+        self.end_headers()
+        self.wfile.write("It works!")
+
+    def version_string(self):
+        return "1.0"
+
+    def date_time_string(self):
+        return "July 22, 2013"
+
+
+if __name__ == "__main__":
+    from optparse import OptionParser
+    p = OptionParser()
+    p.add_option("-a", "--addr", type="string", default="localhost",
+                 help=("listen on given address (numeric IP or host name), "
+                       "an empty string (the default) means INADDR_ANY"))
+    p.add_option("-p", "--port", type="int", default=32123,
+                 help="listen on given TCP port number")
+    p.add_option("-m", "--max", type="int", default=-1,
+                 help="max number of requests to respond to, -1 means no max")
+    options, args = p.parse_args()
+
+    httpd = BaseHTTPServer.HTTPServer((options.addr, options.port),
+                                      MyRequestHandler)
+    if options.max == -1:
+        httpd.serve_forever()
+    else:
+        served_count = 0
+        while served_count != options.max:
+            httpd.handle_request()
+            served_count += 1