Support dub's actual (and special) `dub test` configuration

[kinke]

dub test adds a special test configuration if there's either no unittest config, or its targetType isn't executable. That special config is derived from the unittest (or default) configuration, incl. generating and compiling-in an extra dub_test_root module with a main() entry point (and an AliasSeq of all root project modules). See https://github.com/dlang/dub/pull/2218 for details. That PR landed after the dub v1.29 branch-off, so will land in v1.30. https://github.com/dlang/dub/pull/2288 and https://github.com/dlang/dub/pull/2300 are additionally required for reggae and will land in v1.30 too.