Issue #11277: Add tests for mmap crash when using large sparse files on OS X.
Issue #11277: Add tests for mmap crash when using large sparse files on OS X.

Also, reduce code duplication in LargeMmapTests.

Original patch by Steffen Daode Nurpmeso.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator