Test Runner: Add stub register function to white box tests. These tests use the same main function as end2end_tests. Add the register function stub so the compilation works correctly in standalone and component builds. Bug: angleproject:4449 Change-Id: Ib1558a596b9a558b16a7c5b7bfccc1e495a67e39 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2082135 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>