Lower the size allocated in the "corrupted input" unit test from 256 MB
to 2 MB. This fixes issues with running the unit test on platforms with
little RAM (e.g. some ARM boards).

Also, reactivate the 2 MB test for 64-bit platforms; there's no good
reason why it shouldn't be.

R=sanjay


git-svn-id: https://snappy.googlecode.com/svn/trunk@58 03e5f5b5-db94-4691-08a0-1a8bf15f6143