devel: simplify ngx_pagespeed development flow (#1363)

* provide scripts/rebuild.sh so you don't have to run make commands in
  two directories
* make test/run_tests.sh able to run with no arguments by setting good defaults.

scripts/rebuild.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+#
+# Copyright 2016 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Usage: scripts/rebuild.sh
+#
+# After building with "scripts/build_ngx_pagespeed.sh --devel", if you make
+# changes to ngx_pagespeed you'll need to rebuild it.  The underlying commands
+# aren't complicated, but it's faster to work if it's automated.
+
+set -e  # exit script if any command returns an error
+set -u  # exit the script if any variable is uninitialized
+
+this_dir="$(dirname $0)"
+cd "$this_dir/.."
+nps_dir="$PWD"
+
+cd "$nps_dir/testing-dependencies/mod_pagespeed/devel"
+make apache_debug_psol
+
+cd "$nps_dir/testing-dependencies/nginx/"
+make
+make install