[GSoC][t2400] Use test_path_is_* functions in test scripts

[Sitao Chen <sitao977g@xxxxxxxxx>]

Hi,

My name is Sitao. I am a CS Master. This is my first time applying
GSoC. I am currently working on replacing ‘test -(e|f|d|…)’ with the
corresponding helper functions in t2400-worktree-add.sh. Let me know
if you have any comments or anything else!

Best,
Sitao