[tests] Updates for the change in path.

These files were moved from the root directory to lib/ in
https://dart-review.googlesource.com/c/sdk/+/405140, but the tests
that used them weren't updated accordingly.

TEST=service/breakpoint_in_package_parts_class_file_uri_test

Fixes: #59980

Change-Id: Ie7eb4666baa7caab95a1aa8469fc890a127f9565
Cq-Include-Trybots: luci.dart.try:vm-linux-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405762
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

runtime/observatory/tests/service/breakpoint_in_package_parts_class_file_uri_test.dart

@@ -14,7 +14,8 @@ import 'service_test_common.dart';
 // Chop off the file name.
 String baseDirectory = path.dirname(Platform.script.path) + '/';
 Uri baseUri = Platform.script.replace(path: baseDirectory);
-Uri breakpointFile = baseUri.resolve('observatory_test_package/the_part.dart');
+Uri breakpointFile =
+    baseUri.resolve('observatory_test_package/lib/the_part.dart');
 const String shortFile = "the_part.dart";
 
 const int LINE = 87;