libtest: Stricter XML validation

Expect the entire input to be a single XML document, instead of
reading it line by line. This detects trailing junk better.

tests/run-make/libtest-junit/validate_junit.py

@@ -13,10 +13,10 @@
 import sys
 import xml.etree.ElementTree as ET
 
-# Try to decode line in order to ensure it is a valid XML document
-for line in sys.stdin:
-    try:
-        ET.fromstring(line)
-    except ET.ParseError:
-        print("Invalid xml: %r" % line)
-        raise
+# Read the entire output and try to decode it as XML.
+junit = sys.stdin.read()
+try:
+    ET.fromstring(junit)
+except ET.ParseError:
+    print("Invalid xml: %r" % junit)
+    raise