Add unittest for issue 22609

[nordlow]

Requested at #8278 (comment).

FYI, @JohanEngelen.

Test copied from https://issues.dlang.org/show_bug.cgi?id=22609.

[dlang-bot]

Thanks for your pull request and interest in making D better, @nordlow! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	22609	regression	[REG2.097] std.format.format fails for opEquals overloads without int

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 + phobos#8339"