use xml-based check for yoda_test_linter

Branch comparison crashing a bunch because of failing on unparseable files -- this is why

R/yoda_test_linter.R

@@ -12,7 +12,7 @@
 #' @export
 yoda_test_linter <- function() {
   Linter(function(source_file) {
-    if (length(source_file$parsed_content) == 0L) {
+    if (length(source_file$xml_parsed_content) == 0L) {
       return(list())
     }