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As mentioned in code review feedback on another spell check CL, the site
isolation browser test file is getting large, and spell check stuff
takes a good chunk of it.

This CL moves the spell check site isolation tests to their own file,
which is not compiled if spell check is not enabled. The CL also moves
a base test class to its own .h and .cc files, since both the non-spell
check and the spell check tests inherit from it.

Other CL for context:
https://chromium-review.googlesource.com/c/chromium/src/+/1918277

Change-Id: Iaea04af280859e42f916667c767a72ddaefcef92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945276
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721120}