Add regression test for alloc stack trace and unloading

Test creates allocations in a different loader, then unloads it.
This would free the ArtMethod except that we now visit them as
roots.

Bug: 26849503

(cherry picked from commit 8a7ef10d10ac1509b79d315fe865bd3c4731262b)

Change-Id: If63e26b49380e7b99244ee25e70e729a1736344a
2 files changed