Disable parallelism on retry

[nedtwigg]

I have a big bank of tests where:

• 100% pass if we run the testsuite single-threaded
• 100% sometimes pass if we run the testsuite in parallel
• 5% of tests on average will fail if we run the testsuite in parallel, due to heavy load

This is good - we can speed up our test suite a ton by going parallel, and then rerun the handful of tests that failed due to load.

But ideally I wish the rerun could do something like:

test {
  useJUnitPlatform {}
  systemProperty 'junit.vintage.execution.parallel.enabled', 'true'
  retry {
    maxRetries = 1
    onRetry {
      systemProperty 'junit.vintage.execution.parallel.enabled', 'false'
    }
  }
}