dmd -unittest=+foo.bar,+std,-std.stdio args... to specify unittests in select pkg/mod

[Timothee Cour]

how about using same syntax (and reusing logic) as newly introduced`
-i=+foo.bar,+baz-baz.bad`:

`dmd -unittest=+foo.bar,+baz,-baz.bad rest_of_arguments`

which would only enable unittests as specified? It's flexible and
intuitive, and would solve a common woe with unittests (eg
https://forum.dlang.org/post/mailman.3165.1517968619.9493.digitalmars-d@puremagic.com)

originally proposed here:
https://forum.dlang.org/post/mailman.3166.1517969180.9493.digitalmars-d@puremagic.com