Reduce FileSystem outerloop test time

Reduce FileSystem outerloop test time

Changes:
- An outerloop test verifying that we would throw when trying to use File.ReadAllBytes to read a > 2GB file was first writing out 2GB worth of data to disk.  We can instead just create a FileStream and explicitly change its length.  On my machine, this changes the execution time of that test from ~25s to just a few milliseconds.
- Two "mini stress" tests are enabled in outerloop and are explicitly coded to run for 30 seconds doing lots of reading and writing.  I've changed these to be 10 seconds.

Commit migrated from https://github.com/dotnet/corefx/commit/e4c1fed1843c2dc80b9b345d6fac0251e865a227