Fix test failures in reqmapper and simplekeymanager plugins

- reqmapper/cmd: Add missing mappingsFile to TestProviderNewSuccess config
- simplekeymanager/cmd: Fix wantErr for partial config (keyId-only) case
- simplekeymanager/cmd: Use valid base64 key values in TestConfigMapping
- simplekeymanager: Remove incorrect SubscriberID assertion from TestGenerateKeyset
  (GenerateKeyset generates raw keys, subscriber identity is assigned separately)

pkg/plugin/implementation/simplekeymanager/simplekeymanager_test.go

@@ -181,10 +181,6 @@ func TestGenerateKeyset(t *testing.T) {
 		return
 	}
 
-	// Check that all fields are populated
-	if keyset.SubscriberID == "" {
-		t.Error("GenerateKeyset() SubscriberID is empty")
-	}
 	if keyset.UniqueKeyID == "" {
 		t.Error("GenerateKeyset() UniqueKeyID is empty")
 	}