ueventd: add tests for setegid()/setfscreatecon() and threads

setegid() and setfscreatecon() on Android both operate on a per-thread
basis, not a per-process basis.

Ueventd may take advantage of this in the future, so this CL
introduces tests that ensure that this functionality remains
consistent.

Bug: 63441941
Test: newly added unit tests
Change-Id: I8b1c62cc322b6fe44b748550a4cea8658d9efd88