tests(junit-xml): avoid invalid XML

When a test case is run in a subshell, we finalize the JUnit-style XML
when said subshell exits. But then we continue to write into that XML as
if nothing had happened.

This leads to Azure Pipelines' Publish Test Results task complaining:

	Failed to read /home/vsts/work/1/s/t/out/TEST-t0000-basic.xml.
	Error : Unexpected end tag. Line 110, position 5.

And indeed, the resulting XML is incorrect.

Let's "re-open" the XML in such a case, i.e. remove the previously added
closing tags.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/test-lib.sh

@@ -1085,6 +1085,7 @@ finalize_junit_xml () {
 		junit_time=$(test-tool date getnanos $junit_suite_start)
 		sed -e "s/\(<testsuite.*\) time=\"[^\"]*\"/\1/" \
 			-e "s/<testsuite [^>]*/& time=\"$junit_time\"/" \
+			-e '/^ *<\/testsuite/d' \
 			<"$junit_xml_path" >"$junit_xml_path.new"
 		mv "$junit_xml_path.new" "$junit_xml_path"