[PDF] Make form field focus test cases more realistic

Currently, PDFiumEngineInkTest.KillFormFocusInAnnotationMode checks to
see if the form field focus gets killed, but the test starts out with
the form field not focused. To make the test case more realistic, first
tab into a form field, so killing focus actually does something
meaningful. To do this easily, change PDFiumEngineInkTest to inherit
from PDFiumEngineTabbingTest.

Make the same change to PDFiumEngineReadOnlyTest.KillFormFocus and
PDFiumEngineTest.SetFormHighlight. Move the SetFormHighlight test into
the PDFiumEngineTabbingTest fixture.

Change-Id: I1038842ef5849dbb74de0a1bf94daa816280babd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7007355
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524869}