Show failures that are ignored due to whitelisting as skipped checks in JUnit output

Continue to show (unwhitelisted) failed checks as failures in JUnit output, but rather than exclude failing whitelisted checks from JUnit, mark them as skipped

Fixes #590

include/junit_integration

@@ -76,6 +76,10 @@ output_junit_failure() {
   output_junit_test_case "$1" "<failure message=\"$(xml_escape "$1")\"/>"
 }
 
+output_junit_skipped() {
+  output_junit_test_case "$1" "<skipped message=\"$(xml_escape "$1")\"/>"
+}
+
 get_junit_classname() {
   # <section>.<check_id> naturally follows a Java package structure, so it is suitable as a package name
   echo "$TITLE_ID"