Projects supporting extensions that used the test HTTP client would end

up reloading all extensions, creating RegisteredExtension entries, every

time a request was performed. This added up quite a bit over time.
This change does two things to address this problem.
1. The RegisteredExtension creation code has been improved. Instead of

   using get_or_create, we attempt a create and fall back on a get. This

   chops the number of queries in half, since we no longer are assuming

   there's an entry in the database already (which is a bad assumption,

   since earlier in the function, we checked and didn't see any).
While here, I also fixed a bug where the wrong value was being

   used for the stored extension name, due to a scoping issue.
2. ExtensionsMiddleware no longer performs expiration checks if

settings.RUNNING_TEST is True. It's up to the projects to set this

   for the test run, though.

Ran the Review Board test suite. It went down from ~147.78 seconds to

~125.45 seconds -- a ~22 second savings. That's with 9 extensions

installed (not enabled) on the system.