testsuite: add borg1_hardlink_master/slave/hardlinkable unit tests, refs #9556

[mr-raj12]

Description

Unit tests for borg1_hardlinkable, borg1_hardlink_master, and borg1_hardlink_slave, refs #9556.

legacy/helpers.py has three predicates that UpgraderFrom12To20.upgrade_item() uses to tell hardlink masters from slaves from regular items. PR #9675 tests the upgrade flow end to end, but these predicates themselves had no coverage. This PR adds src/borg/testsuite/legacy_helpers_test.py with 13 tests covering all three.

src/borg/testsuite/legacy_helpers_test.py -- 13 tests:

borg1_hardlinkable (7 tests):

• regular file → True
• block device → True
• char device → True
• FIFO → True
• symlink → False
• directory → False
• socket → False

borg1_hardlink_master (3 tests):

• all conditions met → True
• hardlink_master key absent → False
• hardlink_master=False → False

borg1_hardlink_slave (3 tests):

• source present, regular file → True
• source present, FIFO (non-regular hardlinkable type) → True
• no source → False

Refs #9556

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests (this PR is the tests)
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.95%. Comparing base (7f43779) to head (de5590f).
⚠️ Report is 17 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9685      +/-   ##
==========================================
+ Coverage   83.87%   83.95%   +0.07%     
==========================================
  Files          93       93              
  Lines       15586    15586              
  Branches     2335     2335              
==========================================
+ Hits        13073    13085      +12     
+ Misses       1785     1775      -10     
+ Partials      728      726       -2     

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