Add unit tests for spack help command (#6779)

* Add unit tests for spack help command
* Work around issue with using `spack help` twice in tests

lib/spack/spack/test/cmd/help.py

@@ -0,0 +1,75 @@
+##############################################################################
+# 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/spack/spack
+# Please also see the NOTICE and LICENSE files 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
+##############################################################################
+import pytest
+
+from spack.main import SpackCommand
+
+
+@pytest.mark.xfail
+def test_reuse_after_help():
+    """Test `spack help` can be called twice with the same SpackCommand."""
+    help_cmd = SpackCommand('help')
+    help_cmd()
+
+    # This second invocation will somehow fail because the parser no
+    # longer works after add_all_commands() is called in
+    # SpackArgumentParser.format_help_sections().
+    #
+    # TODO: figure out why this doesn't work properly and change this
+    # test to use a single SpackCommand.
+    #
+    # It seems that parse_known_args() finds "too few arguments" the
+    # second time through b/c add_all_commands() ends up leaving extra
+    # positionals in the parser. But this used to work before we loaded
+    # commands lazily.
+    help_cmd()
+
+
+def test_help():
+    """Sanity check the help command to make sure it works."""
+    help_cmd = SpackCommand('help')
+    out = help_cmd()
+    assert 'These are common spack commands:' in out
+
+
+def test_help_all():
+    """Test the spack help --all flag"""
+    help_cmd = SpackCommand('help')
+    out = help_cmd('--all')
+    assert 'Complete list of spack commands:' in out
+
+
+def test_help_spec():
+    """Test the spack help --spec flag"""
+    help_cmd = SpackCommand('help')
+    out = help_cmd('--spec')
+    assert 'spec expression syntax:' in out
+
+
+def test_help_subcommand():
+    """Test the spack help subcommand argument"""
+    help_cmd = SpackCommand('help')
+    out = help_cmd('help')
+    assert 'get help on spack and its commands' in out