Expose and document `psutil.test.MemoryLeakTestCase`

[MemoryLeakTestCase](https://github.com/giampaolo/psutil/blob/a07e87a1336678cbcc929116cd826137779cf654/psutil/tests/__init__.py#L1158) class is quite useful, to the point that I think it's worth promoting it to an official API, and document it. 