linter: convert path in test infra

Change-Id: Ie8c61aae0dc20be53c26741ef98f67f52c928bfd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367201
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Auto-Submit: Samuel Rawlins <srawlins@google.com>
diff --git a/pkg/linter/test/rule_test_support.dart b/pkg/linter/test/rule_test_support.dart
index 3e37672..821d697 100644
--- a/pkg/linter/test/rule_test_support.dart
+++ b/pkg/linter/test/rule_test_support.dart
@@ -338,7 +338,7 @@
       List<(String path, List<ExpectedDiagnostic> expectedDiagnostics)>
           unitsAndDiagnostics) async {
     for (var (path, expectedDiagnostics) in unitsAndDiagnostics) {
-      result = await resolveFile(path);
+      result = await resolveFile(convertPath(path));
       await assertDiagnosticsIn(result.errors, expectedDiagnostics);
     }
   }