RFR: 8255548: Missing coverage for javax.xml.crypto.dom.DOMCryptoContext

[Matthew Donovan]

Implemented tests for the `DomCryptoContext.iterator()` method.

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Commit messages:
 - 8255548: Missing coverage for javax.xml.crypto.dom.DOMCryptoContext

Changes: https://git.openjdk.org/jdk/pull/13445/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13445&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8255548
  Stats: 46 lines in 1 file changed: 45 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13445.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13445/head:pull/13445

PR: https://git.openjdk.org/jdk/pull/13445