[svsim] Make EphemeralSimulator multi-processing friendly

[jackkoenig]

I and other users have noticed rare hangs, I am not certain this fixes them but I believe it will. I also noticed on shared machines that another user owned the directory the EphemeralTester was trying to put stuff in so I could no longer run sbt 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

• Bugfix

Desired Merge Strategy

• Squash

Release Notes

• Grab a unique temporary directory for every invocation using Java API
• Allow multiple EphemeralSimulators to run in the same JVM (no longer necessary to single-thread)
• Use pure Scala code to recursively delete directory at end rather than platform specific rm -rf

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), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[seldridge]

LGTM