With DMD, disabling the invariants for the unittest runner is required
to overcome a missing symbol (an invariant) for dub's vendored
`std.sumtype` copy.

Don't rely on the Phobos `dirEntries()` order; it's e.g. problematic
for the `tests.it.rules.static_lib.template` test.

…ld` with DMD on Windows

As I've just recently hit long-path issues on Windows for this reason,
especially when the absolute path of the main package is a bit longer.

Also skip the `__dub__` intermediate directory, as all dub packages
now produce object files in `<target .obj dir>/<dub package name>/…`.