[infra] Handle test steps with no tested configuration

Change-Id: I95543f88a64884bf2334165baa986d381bbf7872
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141241
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Karl Klose <karlklose@google.com>

tools/test.dart

diff --git a/tools/test.dart b/tools/test.dart
index aa8e9f6..dfcbf5d 100755
--- a/tools/test.dart
+++ b/tools/test.dart
@@ -112,7 +112,13 @@
     }
     foundBuilder = true;
     for (final step in builder.steps.where((step) => step.isTestStep)) {
-      final namedConfiguration = step.testedConfiguration.name;
+      final testedConfiguration = step.testedConfiguration;
+      if (testedConfiguration == null) {
+        // This test step does not use a configuration; for example,
+        // because it is a simple script that does not produce results.
+        continue;
+      }
+      final namedConfiguration = testedConfiguration.name;
       if (requestedNamedConfigurations.isEmpty ||
           requestedNamedConfigurations.contains(namedConfiguration)) {
         testedConfigurations