Some MerkleDb unit tests take too long to run

[artemananiev]

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👾 Description of the issue

I ran a clean build (./gradlew clean; ./gradlew build) locally, it took 12 mins, out of which more than 4 mins were spent in :swirlds-merkledb:test:

> Task :swirlds-merkledb:test
  739 passing (4m 5s)

In particular, HalfDiskVirtualKeySetTest took a minute or so, and there were a couple more slow tests. Some of the tests are named "hammer test", which indicates they should be tagged appropriately, but they aren't:

    @Tags({@Tag(TestTypeTags.HAMMER), @Tag(TestQualifierTags.TIME_CONSUMING)})

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