Add test case for Issue 21390

[thewilsonator]

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[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21390	minor	Cannot declare extern (storage class) variable of type with disabled default construction

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#13733"