Unit tests for webassets

remove the unit tests for the lesscss generator (TestLessCSSGenerator class) and
replace it with tests for webassets:

- add a sample scss file
- run pelican with example files
- compare the generated css file with a reference one
- look in the html code for the correct link tag

tests/themes/assets/static/css/style.min.css

@@ -0,0 +1 @@
+body{font:14px/1.5 "Droid Sans",sans-serif;background-color:#e4e4e4;color:#242424}a{color:red}a:hover{color:orange}

tests/themes/assets/static/css/style.scss

@@ -0,0 +1,19 @@
+/* -*- scss-compile-at-save: nil -*- */
+
+$baseFontFamily : "Droid Sans", sans-serif;
+$textColor      : #242424;
+$bodyBackground : #e4e4e4;
+
+body {
+  font: 14px/1.5 $baseFontFamily;
+  background-color: $bodyBackground;
+  color: $textColor;
+}
+
+a {
+  color: red;
+
+  &:hover {
+    color: orange;
+  }
+}

tests/themes/assets/templates/base.html

@@ -0,0 +1,7 @@
+{% extends "!simple/base.html" %}
+
+{% block head %}
+  {% assets filters="scss,cssmin", output="gen/style.%(version)s.min.css", "css/style.scss" %}
+    <link rel="stylesheet" href="{{ ASSET_URL }}">
+  {% endassets %}
+{% endblock %}