[test] Remove extra / in dartdevc nndb test suite runner.

Change-Id: I0f04cb2414b726005a60b4a2263be1bacd5ddb1f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127643
Reviewed-by: Alexander Thomas <athom@google.com>

Author: whesse

pkg/dev_compiler/test/modular_suite_nnbd.dart

@@ -21,7 +21,7 @@ main(List<String> args) async {
   await _resolveScripts();
   await runSuite(
       sdkRoot.resolve('tests/compiler/dartdevc/modular/'),
-      'tests/compiler/dartdevc/modular/',
+      'tests/compiler/dartdevc/modular',
       _options,
       IOPipeline([
         SourceToSummaryDillStep(),