Prefer the host dmd to compile the test runner

[MoonlightSentinel]

This ensures that all tests are run properly and potential failures (e.g. because of faulty codegen) occur in small snippets instead of the test runner itself.

[dlang-bot]

Thanks for your pull request and interest in making D better, @MoonlightSentinel! 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.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#10950"

[Geod24]

Maybe use a slightly longer name here ?

[Geod24]

Or not, since it was merged just now.

[MoonlightSentinel]

Sure, I'll fix this in another PR.

[MoonlightSentinel]

See #10977