Declare precise node to run node-specific tests

[MarkEWaite]

Declare precise node to run node-specific tests

Tests may have been passing by accident or luck, since the precise node was not specified and the tests depend on a precise node.

Also extends allowed file system offset in test

Busy file systems can be even slower than estimated. I needed an additional 1 second offset when Jenkins core tests were running on the same Linux computer that was testing the git plugin.

Testing done

Confirmed tests pass consistently on my Linux development computer.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed