[lint] Check variants for reftests (web-platform-tests#42129)

As of web-platform-tests#40352, reftests may use variants and should be linted too. All
files still lint cleanly, according to `wpt lint --all`.

Fixes https://crbug.com/1485860#c1

tools/lint/lint.py

@@ -433,6 +433,18 @@ def check_parsed(repo_root: Text, path: Text, f: IO[bytes]) -> List[rules.Error]
         if timeout_value != "long":
             errors.append(rules.InvalidTimeout.error(path, (timeout_value,)))
 
+    if source_file.content_is_ref_node or source_file.content_is_testharness:
+        for element in source_file.variant_nodes:
+            if "content" not in element.attrib:
+                errors.append(rules.VariantMissing.error(path))
+            else:
+                variant = element.attrib["content"]
+                if (variant == "" or
+                    variant[0] not in ("?", "#") or
+                    len(variant) == 1 or
+                    (variant[0] == "?" and variant[1] == "#")):
+                    errors.append(rules.MalformedVariant.error(path, (path,)))
+
     required_elements: List[Text] = []
 
     testharnessreport_nodes: List[ElementTree.Element] = []
@@ -450,17 +462,6 @@ def check_parsed(repo_root: Text, path: Text, f: IO[bytes]) -> List[rules.Error]
             if len(testharnessreport_nodes) > 1:
                 errors.append(rules.MultipleTestharnessReport.error(path))
 
-        for element in source_file.variant_nodes:
-            if "content" not in element.attrib:
-                errors.append(rules.VariantMissing.error(path))
-            else:
-                variant = element.attrib["content"]
-                if (variant == "" or
-                    variant[0] not in ("?", "#") or
-                    len(variant) == 1 or
-                    (variant[0] == "?" and variant[1] == "#")):
-                    errors.append(rules.MalformedVariant.error(path, (path,)))
-
         required_elements.extend(key for key, value in {"testharness": True,
                                                         "testharnessreport": len(testharnessreport_nodes) > 0,
                                                         "timeout": len(source_file.timeout_nodes) > 0}.items()