test(junit): avoid line feeds in XML attributes

test(junit): avoid line feeds in XML attributes

In the test case's output, we do want newline characters, but in the XML
attributes we do not want them.

However, the `xml_attr_encode` function always adds a Line Feed at the
end (which are then encoded as `
`, even for XML attributes.

This seems not to faze Azure Pipelines' XML parser, but it still is
incorrect, so let's fix it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>