Meta: Disallow importing ref tests with multiple match or mismatch links

Our internal test runner does not currently support this, so let's
explicitly disallow it for now to avoid potential confusion.

Meta/import-wpt-test.py

@@ -79,13 +79,17 @@ class TestTypeIdentifier(HTMLParser):
         self.url = url
         self.test_type = TestType.TEXT
         self.reference_path = None
+        self.ref_test_link_found = False
 
     def handle_starttag(self, tag, attrs):
         if tag == "link":
             attr_dict = dict(attrs)
             if attr_dict["rel"] == "match" or attr_dict["rel"] == "mismatch":
+                if self.ref_test_link_found:
+                    raise RuntimeError("Ref tests with multiple match or mismatch links are not currently supported")
                 self.test_type = TestType.REF
                 self.reference_path = attr_dict["href"]
+                self.ref_test_link_found = True
 
 
 def map_to_path(sources, is_resource=True, resource_path=None):