Add a test for building the dmd frontend with dmd -i

[wilzbach]

Building the dmd frontend with dmd -i already works, so better add a simple CI check to ensure that it continues to work.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[jacob-carlborg]

The PR, the commit message and the comment mention "rdmd" but I don't see rdmd being used.

[wilzbach]

Sorry I used rdmd initially, but then realized that we don't even need rdmd here, but can use vanilla dmd with the new -i feature.