Add SearchSuggestionOnLocalNtp to field trial testing config

Add last feature to UseGoogleLocalNtp experiment.

Bug: None
Change-Id: I06e3ef9caded7fc19735ca0dbf44bc2ea97098a7
Reviewed-on: https://chromium-review.googlesource.com/c/1470975
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631950}

testing/variations/fieldtrial_testing_config.json

@@ -5054,6 +5054,7 @@
                     "enable_features": [
                         "DoodlesOnLocalNtp",
                         "PromosOnLocalNtp",
+                        "SearchSuggestionsOnLocalNtp",
                         "UseGoogleLocalNtp"
                     ]
                 }