rz-test: Set default test dir to '.'

[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 sets rz-test's default test dir to '.'. This allows for easy running of local test files outside the official test hierarchy, via rz‑test ./<testfile> or rz‑test <testfile>.

Test plan

The change makes sense. All builds are green.

Closing issues

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