tests: add tests for zkfs utilities

[EshaanAgg]

Description

• Added the tests for some utilities for vitess/go/cmd/zk/internal/zkfs
• I couldn't add the complete coverage code as I am having trouble mocking the zk2topo.ZkConn in a simple manner for testing. If anyone can point me to any files that might have done the same previously, or give me some clue as to how we might go about it (without defining all the required methods explicitly), I would love to update the PR to handle the same as well.

Related Issue(s)

Fixes part of #14931

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9af8692) 47.33% compared to head (0fcf01a) 47.33%.
Report is 72 commits behind head on main.

❗ Current head 0fcf01a differs from pull request most recent head deed3a9. Consider uploading reports for the commit deed3a9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #15002    +/-   ##
========================================
  Coverage   47.33%   47.33%            
========================================
  Files        1142     1143     +1     
  Lines      239009   239109   +100     
========================================
+ Hits       113125   113191    +66     
- Misses     117289   117321    +32     
- Partials     8595     8597     +2     

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