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/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 %}