We have unit tests that involve static media bundles, but they're not

really involving any actual files in those bundles, meaning that certain

logic never gets instrumented.
This change adds the settings and support needed to be able to involve

actual static files in tests. It does this by adding in several static

files (source and bundled, both raw and versioned), so that they can be

looked up through the static media system.
Since we're not using actual extensions backed by a Python package, we

needed to be able to fake results for static media lookups. Internally,

ExtensionManager used pkg_resources methods to perform lookups in

packages to get this information, and spying on those methods was one

possible way of doing this, but instead we're adding methods to

ExtensionInfo for performing these lookups. This abstracts much of the

actual package access away, and gives us a clearer target for spying

(and, down the road, we could make it possible to provide a subclass of

ExtensionInfo more specific for testing purposes).
Some tests had to change in order to test against versioned files.

Unit tests pass on all supported versions of Python.