ci: avoid optional liquidSVM in coverage deps

[alceops]

Fixes #10.

Summary

• keep the coverage workflow from trying to install every optional Suggests package
• explicitly install testthat for the coverage test runner
• leave package code/tests unchanged

The current test-coverage.yaml run fails before tests start during setup-r-dependencies, because pak tries to solve optional Suggests and cannot find liquidSVM:

* deps::.: Can't install dependency liquidSVM
* liquidSVM: Can't find package called liquidSVM.

liquidSVM is documented as optional in the README and the package already has fallback behavior for non-liquidSVM methods, so the CI dependency install should avoid requiring it just to run coverage.

Verification

• git diff --check ✅
• Inspected the latest failing Actions log for run 24883441814; failure is in r-lib/actions/setup-r-dependencies@v2 before coverage/tests execute.
• Local R verification not run in this worker because R is not installed here.

Safety

Workflow-only change. No secrets, deployments, external services, or package runtime behavior changed.