[test] Fix bug in export/duplicate_import_libd

`git show fc1b1ecc712d` shows - among lots of other changes - this:

```
diff --git a/tests/language_2/duplicate_import_liba.dart b/tests/language_2/export/duplicate_import_liba.dart
similarity index 100%
rename from tests/language_2/duplicate_import_liba.dart
rename to tests/language_2/export/duplicate_import_liba.dart
```

and

```
diff --git a/tests/language_2/duplicate_import_libd.dart b/tests/language_2/export/duplicate_import_libd.dart
similarity index 91%
rename from tests/language_2/duplicate_import_libd.dart
rename to tests/language_2/export/duplicate_import_libd.dart
index ccec81e0ff6..14f1ff413fb 100644
--- a/tests/language_2/duplicate_import_libd.dart
+++ b/tests/language_2/export/duplicate_import_libd.dart
@@ -8,6 +8,6 @@ import 'duplicate_import_liba.dart';
 import 'duplicate_import_libb.dart';
 import 'duplicate_import_libc.dart';

-export 'duplicate_import_liba.dart';
+export '../duplicate_import_liba.dart';
 export 'duplicate_import_libb.dart';
 export 'duplicate_import_libc.dart';
```

Change-Id: I806ba94ae251d574981e06ea88e6189063719785
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205264
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
diff --git a/tests/language/export/duplicate_import_libd.dart b/tests/language/export/duplicate_import_libd.dart
index 14f1ff4..ccec81e 100644
--- a/tests/language/export/duplicate_import_libd.dart
+++ b/tests/language/export/duplicate_import_libd.dart
@@ -8,6 +8,6 @@
 import 'duplicate_import_libb.dart';
 import 'duplicate_import_libc.dart';
 
-export '../duplicate_import_liba.dart';
+export 'duplicate_import_liba.dart';
 export 'duplicate_import_libb.dart';
 export 'duplicate_import_libc.dart';
diff --git a/tests/language_2/export/duplicate_import_libd.dart b/tests/language_2/export/duplicate_import_libd.dart
index 54bd35c..3408bfa 100644
--- a/tests/language_2/export/duplicate_import_libd.dart
+++ b/tests/language_2/export/duplicate_import_libd.dart
@@ -10,6 +10,6 @@
 import 'duplicate_import_libb.dart';
 import 'duplicate_import_libc.dart';
 
-export '../duplicate_import_liba.dart';
+export 'duplicate_import_liba.dart';
 export 'duplicate_import_libb.dart';
 export 'duplicate_import_libc.dart';