Change the IOperation unit test dump format for child nodes

[mavasani]

Currently, the dump prints the first child on the same line as the children header, which makes reviewing the dump very tedious. @AlekseyTs and @jcouv have both requested to change this format to dump the first child node on a new line.

It is a single line change in the dumper: we just need to append Environment.NewLine to the string argument passed to this LogString call: http://source.roslyn.io/#Roslyn.Test.Utilities/Compilation/OperationTreeVerifier.cs,245

However, this will need to update the expected output for almost all of the existing IOperation unit tests. We should do this sooner rather than later though.

@jinujoseph Can we get the vendors do this task as soon as possible?

[jinujoseph]

closed via #20716