[roottest] root/tree/branches/runseparate.C fails on Windows

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Description

When converting to cmake, find out that macro fails.
Problem when after opening and closing created treeobj.root file extra branch.root file left opened.
Therefore getting extra output compared to ref file.

Reproducer

Just run macro to see that extra output with "Some files where not closed" appeared.

ROOT version

master on Windows

Installation method

build from source

Operating system

Win 64bit

Additional context

Recognized by testing in #19516