Fix ConcurrentDictionary test by rooting members accessed via reflection

[stephentoub]

Fixes#81945

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Issue Details

---

Fixes #81945

Author:	stephentoub
Assignees:	-
Labels:	area-System.Collections
Milestone:	-

[stephentoub]

/azp list

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[stephentoub]

/azp run runtime-extra-platforms

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[eiriktsarpalis]

If CI is happy LGTM

[MichalStrehovsky]

FWIW, this test would not require the RD.XML if the test replaced cd.GetType().GetField("_tables", BindingFlags.Instance | BindingFlags.NonPublic) and similar with typeof(ConcurrentDictionary<string, string>).GetField("_tables", BindingFlags.Instance | BindingFlags.NonPublic) (if they're equivalent - they might in this case?).

The reflection analysis only analyzes foo.GetType() if foo is sealed.

typeof(X).GetField("literal") works all the time.