test: add table-based tests for resolveAgainstBase

[naman79820]

Description

Adds unit tests for the resolveAgainstBase function in pkg/unikontainers/utils.go, which previously had no test coverage. The tests are added to the existing pkg/unikontainers/utils_test.go (rather than a new file) and are written as table-based tests, following the style already used in vaccel_test.go.

resolveAgainstBase(base, path) resolves path relative to base and returns an absolute path. The tests cover three success cases and one failure case:

• Absolute path — returned unchanged.
• Relative path with absolute base — joined onto the base (e.g. /var/lib/urunc + rootfs/image → /var/lib/urunc/rootfs/image).
• Relative path with relative base — the base is first resolved against the working directory, then joined. The test asserts the exact resolved absolute path rather than a substring.
• Failure case — the function only errors when it cannot resolve a relative base to an absolute path (i.e. when os.Getwd() fails). The test forces this by changing into a temporary directory and removing it, then asserts an error. This case is intentionally not run with t.Parallel() since it mutates the process working directory, and it restores the original working directory afterwards.

This implements the review feedback left on #592 for this function: update the existing test file instead of creating a new one, use table-based tests, add a failure case, assert the exact path instead of using Contains, and drop the hardcoded personal path.

This builds on the work started in #592. Since that PR went stale, I've implemented the maintainer's review feedback in a fresh PR, with credit to her for the original test cases.

Related issues

• Fixes Add more unit tests #96

How was this tested?

Tested locally on WSL with Go 1.26:

• go test ./pkg/unikontainers/ -run TestResolveAgainstBase -v — all cases pass, including the failure case.
• go test ./pkg/unikontainers/ -count=2 — the whole package passes twice, confirming the failure test restores the working directory and doesn't disturb other tests.
• gofmt -l clean, go vet ./pkg/unikontainers/ clean, go build ./... succeeds.

LLM usage

Claude Opus 4.8 assisted with writing the tests and understanding the code. Every line was reviewed, understood, and tested before submission.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[naman79820]

@cmainas :))