[PATCH v5] tests: use test_path_is_missing instead of '! test -f'

Replace a raw '! test -f' check with test_path_is_missing.

The test_path_is_missing helper integrates with Git’s test framework and produces clearer failure output. In contrast, a plain shell '! test -f' check only reports a generic failure status, which makes it harder to understand whether the file unexpectedly exists or if another issue caused the test to fail.

It also avoids relying on negated shell conditions, making the test easier to read and understand.

Signed-off-by: Jayesh Daga <jayeshdaga99@gmail.com>
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v5:
- Clarify rationale for using test helper
- Explain diagnostic improvement and negation issues
- Address review comments on vague wording

v4:
- Correct commit message to match actual change
- Improve rationale (diagnostics, consistency)
- Move version notes below '---'
- Fix author name to match sign-off

v3:
- Fix commit message wording
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 t/pack-refs-tests.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)