[Autofill] Clean up FormStructureRationalizerTest

This CL reduces the redundant code in
form_structure_rationalizer_unittest.cc.
This CL is a no-op.

Bug: 1007974
Change-Id: I0d477802a8083547e5ee191d3045f6b1eb8b7647
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4328137
Commit-Queue: Christoph Schwering <schwering@google.com>
Auto-Submit: Christoph Schwering <schwering@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1115761}