Enable sandbox for testing and run tests in parallel

[unlsycn]

com-lihaoyi/mill#3347 enables the sandbox for testing by default, and this prevents us from assuming that we are running tests in the workspace root. Instead, we should use the environment variables provided by mill to locate the resources that files in tests should access.

Notably, a match assertion of sourceline and caret has been removed from SimulatorSpec, as we don't have an option to specify the source root of this test, it wouldn't undermine the correctness of the test.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).
• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.