WebUI: Migrate CustomizeChrome side panel tests to WebUIMochaBrowserTest.

WebUIMochaBrowserTest is the new way of running WebUI tests without any
dependency to the obsolete js2gtest and test_api.js mechanism.

Also doing minor renaming of tests, now that previous js2gtest
constraint of having a dedicated C++ class for each test case does not
exist and and removing the somewhat meaningless '*.All' naming pattern.
Example renames:
 - SidePanelCustomizeChromeButtonLabelTest.All ->
   SidePanelCustomizeChromeTest.ButtonLabel
 - SidePanelCustomizeChromeCardsTest.All ->
   SidePanelCustomizeChromeTest.ChromeCards

which also makes running such tests much easier by specifying
--gtest_filter=SidePanelCustomizeChromeTest.* instead of
--gtest_filter=SidePanelCustomizeChrome*Test.All

Bug: 1457360
Change-Id: Ie621175a346c888a2356eab5ea98cba0fa8fcb8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4679896
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1169702}