Add --all-subpackages option on test command

[shoo]

Related: #1873
I'd rather test subpackages recursively.

This will conflict with #1873. Therefore, further editing is required after merging #1873.
Maybe it needs to have a consistent command name as well.

[dlang-bot]

Thanks for your pull request and interest in making D better, @shoo! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

[andre2007]

I am not sure whether I understand the new functionality. The term recurse confuses me because I thought a dub root package can only have a flat list of sub packages, no hierarchy.

Also if I am not completely wrong, by executing dub test on a root package, the unittests of sub packages will already run today.

[shoo]

I am not sure whether I understand the new functionality. The term recurse confuses me because I thought a dub root package can only have a flat list of sub packages, no hierarchy.

I propose --all-subpackages as the next idea.

Also if I am not completely wrong, by executing dub test on a root package, the unittests of sub packages will already run today.

Unfortunately, it doesn't seem to be the case. The test I wrote shows that.
https://github.com/shoo/dub/blob/7891ab47fd98df0b385ef7800069248dec4df770/test/test-recurse-subpackages.sh#L28

If the root package depends on a subpackage, and build with --build-mode=allAtOnce, the subpackage will be tested as well. And also, in that case, it can test with #1873.
But, the root package does not always depend on subpackages.

---

PS. It doesn't matter much about this pull request, but please let me know for later study. Why are subpackages flat? Is there a design reason for not having a nested structure like sub-sub-package?