Move FormEventLoggerBase tests into a separate file.

The tests are currently in autofill_metrics_unittest.cc. This CL moves
them to a new form_event_logger_base_unittest.cc file.

Since FormEventLoggerBase also emits UKM metrics, some of the UKM
testing functionality defined in autofill_metrics_unittest.cc is needed
in form_event_logger_base_unittest.cc. For this reason, new
ukm_metrics_test_utils.(cc|h) files are created. They contain all the
UKM testing helper functions that are currently part of
autofill_metrics_unittest.cc.

The code is moved 1:1 % renaming the test fixture.

Bug: 1007974
Change-Id: I1110890405782919595d519331fe351b3efcb1fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4253948
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1106147}

components/autofill/core/browser/BUILD.gn

@@ -672,6 +672,8 @@ static_library("test_support") {
     "logging/text_log_receiver.h",
     "metrics/autofill_metrics_test_base.cc",
     "metrics/autofill_metrics_test_base.h",
+    "metrics/ukm_metrics_test_utils.cc",
+    "metrics/ukm_metrics_test_utils.h",
     "mock_autocomplete_history_manager.cc",
     "mock_autocomplete_history_manager.h",
     "mock_iban_manager.cc",
@@ -909,6 +911,7 @@ source_set("unit_tests") {
     "manual_testing_profile_import_unittest.cc",
     "merchant_promo_code_manager_unittest.cc",
     "metrics/autofill_metrics_unittest.cc",
+    "metrics/form_events/form_event_logger_base_unittest.cc",
     "metrics/payments/iban_metrics_unittest.cc",
     "metrics/payments/offers_metrics_unittest.cc",
     "metrics/profile_import_metrics_unittest.cc",