Statically load stylesheets in ListView tests #393

Dynamically loading CSS files is an asynchronous process and our
heuristics for detecting when CSS files are done loading don't appear to
be working consistently on Saucelabs.

Make our tests more robust by statically loading CSS files instead.

Also, added a missing dispose check.
"Namespace" rules in ListView.less #393

Prefix every rule in ListView.less with a special class so that its rules
are only in effect for the tests that need the rules.