Add BDD tests for spawn command

Issue-Link: https://github.com/The-Compiler/qutebrowser/issues/999

tests/integration/features/spawn.feature

@@ -0,0 +1,17 @@
+Feature: :spawn
+
+    Scenario: Running :spawn
+        When I run :spawn -v echo "Hello"
+        Then the message "Command exited successfully." should be shown
+
+    Scenario: Running :spawn with command that does not exist
+        When I run :spawn command_does_not_exist127623
+        Then the error "Error while spawning command: The process failed to start." should be shown
+
+    Scenario: Running :spawn with invalid quoting
+        When I run :spawn """
+        Then the error "Error while splitting command: No closing quotation" should be shown
+
+    Scenario: Running :spawn with url variable
+        When I run :spawn echo {url}
+        Then "Executing echo with args ['about:blank'], userscript=False" should be logged

tests/integration/features/test_spawn.py

@@ -0,0 +1,22 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2015-2016 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser 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
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
+
+import pytest_bdd as bdd
+bdd.scenarios('spawn.feature')
+