Revert "rz-test: Set default test dir to '.' (#4741)"

[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

This pr reverts #4741 since apparently it has code that allows tests in the official Rizin test hierarchy to find their test binaries (in test/bins) no matter what the current working directory is.

Test plan

Given the test/db/cmd/cmd_pd testfile pathname, all of the following should work:

1. rz-test test/db/cmd/cmd_pd in the root folder
2. rz-test db/cmd/cmd_pd in the test/ folder
3. rz-test cmd/cmd_pd in the test/db folder
4. rz-test cmd_pd in the test/db/cmd folder

One cannot prepend ./ to the rz-test arguments above though (except for number 2 for some reason -- rz-test ./db/cmd/cmd_pd in the test/ folder works), but that's an issue for another pr.

Also, all builds should be green.

Closing issues

...

[kazarmy]

Updated the pr header comment

[XVilka]

Thank you for writing the documentation!