add test case for issue 18660

[ghost]

Not dedicated to be merged. ping @Spoov

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	18660	regression	getSymbolsByUDA stops after encountering a function

[wilzbach]

i had tested with DMD ~master. So it's no yet fixed.
...
it's resolved in stable.

-> #6342