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
