[scudo] Allow using a different test main.

Fuchsia already uses a different main function for tests, so allow
anybody to use this mechanism.

Specifically, Android has a test main that allows tests to be run
in isolation and in parallel which speeds up the unit test runs
from ~14 seconds to ~4 seconds.

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D159501