Fix SharedBuffer tests to pass under hwasan.

Allocating the entire address space is sufficiently unreasonable that
hwasan blows you out of the water if you try to do that. Half the
address space feels like it's large enough to be guaranteed to fail
(which is all I think this intended to test) without being quite so
obviously incorrect that tools like hwasan are going to object...

...except hwasan actually has a 2GiB limit. So let's keep the cleanup,
but also just disable these tests if they're running under hwasan for
now.

Bug: https://issuetracker.google.com/171054277
Test: treehugger
Change-Id: I41f3def5c3a43aa7f9ca3a130b4306ebed659d6a