Fix checksumming in Python3; add more fetch tests (#3941)

* Checksum code wasn't opening binary files as binary.

- Fixes Python 3 issue where files are opened as unicode text by default,
  and decoding fails for binary blobs.

* Simplify fetch test parametrization.

* - add tests for URL fetching and checksumming.
- fix coverage on interface functions in FetchStrategy superclass
- add some extra crypto tests.

var/spack/repos/builtin.mock/packages/url-test/package.py

@@ -0,0 +1,35 @@
+##############################################################################
+# Copyright (c) 2013-2017, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class UrlTest(Package):
+    """Mock package that fetches from a URL."""
+    homepage = "http://www.url-fetch-example.com"
+
+    version('test', url='to-be-filled-in-by-test')
+
+    def install(self, spec, prefix):
+        pass