WalkerTest tests the Walker solely, it has no relation to snap remover at all. It's my bad, but as I commented on PR, this PR holds _two_ changes:
* adds Walker ability to perform "breadth-first" walks too
* change for Snap remover _use_ this new feature

This test I added to prove "same numbers" will be found at the end only proves that the old walk-type and new walk-type will both gather same stats (will visit same count if files and directories, etc), so to say, both are "correct".

Snap remover has it's own UT and IT and it verifies that it works and does what it should, and does it correctly, despite the underlying implementation did change.

And no, no previous IT/UT should fail at all, as in this case we deal with plain optimisation (file items are reused for performance, instead of being looked up again). The starting and ending state of tests remained the same, as snap remover did not change it's behaviour.

https://github.com/sonatype/nexus-oss/blob/master/components/nexus-core/src/test/java/org/sonatype/nexus/maven/tasks/DefaultSnapshotRemoverTest.java
https://github.com/sonatype/nexus-oss/blob/master/components/nexus-core/src/test/java/org/sonatype/nexus/maven/tasks/DefaultSnapshotRemoverIT.java