Do not generate debug info for the unit test runner when Optlink is used

[jacob-carlborg]

If Optlink is too old the following error can occur:

Error 45: Too Much DEBUG Data for Old CodeView format

This occurs on some of the auto-tester machines.

[dlang-bot]

Thanks for your pull request and interest in making D better, @jacob-carlborg! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
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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 fetch digger
dub run digger -- build "master + dmd#9355"

[jacob-carlborg]

See #9333 (comment) for reference.