LazySuite._precache is never cleared leading to increased memory consumption

[iainb]

As the title says, LazySuite._precache is never cleared, this means that all Test instances are kept in memory for the life time of a test run. I work with a test suite that has a large number of tests, some of which create a lot of temporary objects and store them on the test class. This leads to nose running out of memory and prevents us from running the whole test suite.

I have fixed the issue with this PR #1054

Though it looks like this project is dead :( It would be great if it could be merged in upstream, I'm sure a lot of people who are running nose 1 will benefit from this change.