Order test descriptor children in place

[mpkorstanje]

Overview

Currently, it is not possible to order children
without first removing all children and then
adding them back in order. E.g:

var children = new ArrayList<>(testDescriptor.getChildren());
Collections.shuffle(children);
children.forEach(testDescriptor::removeChild);
children.forEach(testDescriptor::addChild);

By adding orderChildren(UnaryOperator<List<TestDescriptor>> orderer) it now becomes possible to write:

testDescriptor.orderChildren(children -> {
	Collections.shuffle(children);
	return children;
});

Fixes: #4290

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[mpkorstanje]

LGTM!

[marcphilipp]

@mpkorstanje Thank you! 🙇