RFR: 7903443: Propagate verbose flag to agents

[Christian Stein]

Please review this change to make `jtreg` propagate its verbosity configuration to agents.

This pull request also updates the `JUnitRunner` agent to evaluate the verbosity configuration and act accordingly.
For example, passing `-v:nopass,error,fail` at the command line to `jtreg` will suppress the `STARTED: ...` and `SUCCESSFUL: ...` lines in `.jtr` files for JUnit-based tests.

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Commit messages:
 - Suppress logging of successful tests in `nopass` mode
 - Load and save verbose-related parameter
 - Prevent NPEs
 - Merge branch 'openjdk:master' into 7903443-propagate-verbose-flag-to-agents
 - Keep expected output for the time being
 - 7903443: Propagate verbose flag to agents

Changes: https://git.openjdk.org/jtreg/pull/187/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=187&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903443
  Stats: 131 lines in 6 files changed: 117 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/jtreg/pull/187.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/187/head:pull/187

PR: https://git.openjdk.org/jtreg/pull/187