Remove rz_test_chdir()

[kazarmy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

I've finally realized that the likely reason for the binrz/rz-test/ requirement in rz_test_chdir() is that it assumes an in-source (or in-tree) build, where binaries are placed next to their source when building. Meson only allows out-of-source (or out-of-tree) builds, and additionally we have a meson install step, so the final location of Rizin binaries can't really be used to find the Rizin test dirs.

Thus, this pr removes the not-relevant-to-us rz_test_chdir() code, with the extra benefit that adding ./ to a test filename doesn't result in surprising behavior (as noted in #4759). I've added extra details to the rz_test_chdir_fromtest() Doxygen comment since the code can fall back to the current working directory if a test filename isn't provided i.e. rz-test is invoked without an argument.

Test plan

The above makes sense and there are no significant regressions like what is described in #4759. All builds are green.

Closing issues

...

[kazarmy]

Or maybe this should be kept? One can possibly link to the binaries in the Meson build dir and so this could be useful. Perhaps I should just fix rz_test_chdir() and update the Doxygen comment.

[kazarmy]

Perhaps best to simplify matters for now and add search from binary location later

[kazarmy]

... maybe add search from binary location later