ci: avoid test runner hangs by removing swift test --parallel

[oscarcv]

Summary

Fixes CI jobs hanging during test execution by removing explicit SwiftPM parallel test mode from the CI workflow.

Problem

Some GitHub macOS runners can stall for minutes when running:

swift test --parallel

even when local execution completes normally.

Change

Updated CI test step to run:

swift test

instead of:

swift test --parallel

File changed:

• .github/workflows/ci.yml

Why this works

• Swift Testing already supports parallel execution internally.
• Removing top-level --parallel avoids known runner-level scheduling/hanging behavior in certain environments.
• Keeps test coverage and behavior unchanged while improving CI reliability.

Scope

• CI workflow only.
• No production/runtime code changes.

Validation

• Verified local test run passes with:

swift test

• Change pushed to the PR branch and ready to rerun in Actions.