scripts: pylib: fix twister report handling for junitparser >= 4.0.2

With junitparser 4.0.2, the JunitXml.fromfile() now returns a JUnitXml
instance. In order to access the junit.TestSuite instance, we must use
TestSuite.fromelem() to convert it back.

Signed-off-by: Daniel DeGrasse <ddegrasse@tenstorrent.com>