add unit test setup for cudf_kafka

[jameslamb]

Description

Fixes #15841

Proposes adding a basic unit test setup for cudf_kafka.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Notes for Reviewers

I tried to follow the patterns I saw for custreamz and dask_cudf as closely as possible.

If you're wondering why you don't see the [test] extra added to pyproject.toml in the diff... it's already there:

cudf/python/cudf_kafka/pyproject.toml

Lines 28 to 33 in 8b5ff18

	[project.optional-dependencies]
	test = [
	"pytest-cov",
	"pytest-xdist",
	"pytest<8",
	] # This list was generated by `rapids-dependency-file-generator`. To make changes, edit ../../dependencies.yaml and run `rapids-dependency-file-generator`.

[jameslamb]

/merge