Add /rz-pipe to test .gitignore

[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

Since the CI puts rz-pipe in test/rz-pipe:

rizin/.github/workflows/ci.yml

Lines 190 to 195 in e8aa510

	- name: Checkout rzpipe
	if: matrix.enabled
	uses: actions/checkout@v4
	with:
	repository: rizinorg/rz-pipe
	path: test/rz-pipe

this pr adds /rz-pipe to test/.gitignore.

Test plan

The addition is appropriate.

Closing issues

...