More helpful assertions on memory leak test
Created by: JoshCheek
The test for a memory leak failed on CI https://travis-ci.org/rspec/rspec-core/jobs/61159415, saying "expected result to have changed to 0, but is now 1"
This updates it so that future failures will tell us which one was not garbage collected.
See https://github.com/rspec/rspec-core/pull/1950#issuecomment-98784821 for more details.
I'll look into the issue in more detail soonish (its assessment week, so students are super stressed right now, and I'm doing a lot of mindless coding to keep from catching it).