fix: fix _precache memory leak from LazySuite

[iainb]

LazySuite keeps all a reference to all tests (even those that have been run) under self._precache. When running a large number of tests this can cause nose to use up all available system memory. This makes running the whole test suite impossible.

This change fixes that issue and keeps the behaviour of LazySuite.__nonzero__ intact. All tests pass under python 2.7 that passed prior to the change (there are some for the coverage plugin that didn't and still don't pass).