Test fixes

src/test/java/tests/DetailMessage.java

@@ -62,7 +62,7 @@ final class DetailMessage extends TestDiagnostic {
    */
   static DetailMessage parse(TestDiagnostic input, @Nullable Path rootDirectory) {
     Path file = input.getFile();
-    if (rootDirectory != null && !file.toString().equals("")) {
+    if (rootDirectory != null && file.startsWith(rootDirectory)) {
       // Empty file strings cannot be relativized.
       file = rootDirectory.relativize(file);
     }