Missing IterableAssert.inOnlyPane and SingletonAssert.inWindow methods

[damccorm]

It seems InterableAssert and SingletonAssert should provide identical pane assertions. But

there is no inOnlyPane for InterableAssert and inWindow for SingletonAssert.

Imported from Jira BEAM-13765. Original Jira may contain additional context.
Reported by: marcin.kuthan.

[mkuthan]

It has been already resolved #16668