test: cover printBackupFileLocation stdout and path handling

[ivanauth]

Follow-up to #688 addressing #688 (review).

Adds tests for printBackupFileLocation covering:

• destination - (stdout) → no output
• relative filename → resolved to absolute path on stderr
• absolute filename → printed as-is on stderr

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.77%. Comparing base (37aca55) to head (d83aa44).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #690      +/-   ##
==========================================
+ Coverage   43.72%   43.77%   +0.04%     
==========================================
  Files          38       38              
  Lines        6193     6193              
==========================================
+ Hits         2708     2711       +3     
+ Misses       3203     3201       -2     
+ Partials      282      281       -1     

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