[iOS]Move tests_hook import

Those five files are implementing the functions declared in
ios/chrome/app/tests_hook.h. So this import should appear first and
separately. This is not something `git cl format` (i.e. Clang)
understands. So this CL move the import and place it in a un-formatted
section.

Bug: None
Change-Id: I1e448e1d8b090bcd741c908d875e921233850bce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7032006
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530033}