Clean up tests

[nex3]

From sass/sass#1245:

These tests are getting out of hand. Need to refactor these and do so in a way that makes integration with https://github.com/hcatlin/sass-spec much easier.

     553 test/sass/plugin_test.rb
     849 test/sass/script_test.rb
     906 test/sass/source_map_test.rb
    1444 test/sass/extend_test.rb
    1884 test/sass/functions_test.rb
    2008 test/sass/conversion_test.rb
    3364 test/sass/engine_test.rb
   13391 total