[iOS] Add SearchEngineSettingsTestCaseBase class

This class contains all the helpers for search_engine_settings_egtest.mm and search_engine_settings_choice_screen_egtest.mm.

EGTest for FR and US are renamed to
search_engine_settings_choice_screen_egtest.mm =>
  search_engine_settings_eea_egtest.mm
search_engine_settings_egtest.mm =>
  search_engine_settings_non_eea_egtest.mm

Change-Id: I0656daa44be29939c2a09428eea9bb2a1bd5d1d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5604388
Reviewed-by: Samar Chehade-Lepleux <samarchehade@google.com>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313427}