`dub_test_root` is deprecated, but used by unit test runners like Silly

[ArthaTi]

Silly uses UDAs to name unittests. These UDAs are not normally available at runtime to the test runner, so Silly uses traits to fetch them from all known modules. It uses dub_test_root to do so, which has recently been deprecated.

I don't believe there exists a suitable replacement to access all modules at compile time, so I don't think it is appropriate to deprecate dub_test_root.