Refactor test_introducer in web module tests to use a single base test case class.

[fenn-cs]

It would be great to make consistent use of one base test class in the web module rather than switching between unittest.TestCase and testools.TestCase extended in SyncTestCase and AsyncTestCase

See : src/allmydata/test/web/test_introducer.py

[exarkun]

src/allmydata/test/web is a whole package containing 6300 lines of test code. I suggest narrowing the scope here a little further - for example, to just test_introducer or test_root.

Also, let's make it explicit. The preferred base class should be SyncTestCase?. Where necessary, AsyncTestCase? can be used (but if it is not necessary in a particular case, or if we can easily rewrite some test code to not require it, we should use SyncTestCase?).

[fenn-cs]

Thanks @exarkun that makes sense. I think we should default to SyncTestCase it's used in more places and seems to be the natural choice for a default if we would pick any.