This test-only "run-on-projects" option indicates that the test should run on exactly the projects where its parent build runs. This avoids the situation where a build is configured to *not* run on a project, but some wayward test didn't get the memo and "pulls in" the build anyway. This has been particularly problematic with win64-asan/opt. MozReview-Commit-ID: 8UP6FsXBohy