[WiP] Fixes for StepExecutionIterator

[jglick]

JENKINS-26163 appears to be blocking jenkinsci/ssh-agent-plugin#4.

• a test
• extend API to somehow indicate running block-scoped executions
• implement WaitForConditionStepTest.restartDuringDelay
• make sure TimeoutStepTest checks onResume

@reviewbybees

[jglick]

Conversation with @kohsuke: currently outer StepExecutions are not saved at all. CpsThread.setStep(StepExecution) is fine as is (despite the name a new CpsThread is created for every block step, not just parallel block!), but we need a public final int CpsThread.parentId.

[recena]

@jglick I'm working on: make sure TimeoutStepTest checks onResume. Ok?

[jglick]

@recena sure, just need to convert to use RestartableJenkinsRule, reproduce what I suspect is the bug resulting from a failure of onResume to be called, and @Ignore("TODO …") it.

[jglick]

@recena I did not file this as a branch in origin, so you can either file a PR-on-a-PR, or I can push this into the origin repo to make it easier to collaborate.

[jglick]

#161 from an origin branch.