[Outreachy]Microproject: modernize path checks in t1410-reflog.sh

Hello,

I'm Vedansh and I'm interested in contributing to Git through Outreachy 2025.

I have successfully built Git from source on Ubuntu (via WSL2) and run the test suite. All tests pass.

For my microproject, I'd like to modernize the path checking in t/t1410-reflog.sh by replacing 'test -f' with test_path_is_file in lines 133-136 (in the 'rewind' test).

I found 4 instances that are assertions (part of && chains):
- test -f C
- test -f A/B/E
- ! test -f F
- ! test -f A/G

I've verified these are test assertions, not flow control statements, and the test currently passes on my system.

Is this appropriate for a microproject?

Thanks, Vedansh