ExtensionManager.get_absolute_url() returns the URL pointing to the

extension manager UI, and to do this it uses a standard URL lookup.
However, there's a problem with this. When we initialize an extension,

we add any URLs it defines and we then flush the URL cache, requiring it

to be rebuilt for any new lookups. We do the same thing when we

uninitialize a URL.
In unit tests, we enable and disable extensions all the time, at least

once per test. This means that testing an extension with custom URLs or

an admin UI configuration or database page is terribly slow, as it's

constantly clearing, re-building, and clearing the cache.
Not all unit tests need the cache to be rebuilt. To avoid this,

get_absolute_url() now caches the result after the first lookup (since

the extension manager UI isn't going to be moving around).
This makes a huge difference. One extension's test suite dropped from 8

minutes to 2, just from this cached variable.

Unit tests passed.
Tested with a couple extensions and saw significant improvements to test

speed (averaging a 75% reduction in test time).